Board definition for CH32X033F8P6 TSSOP20 #171

At the current stage it works for me. Have fun!
Info below is also in the readme of the board definition directory.

+------v------+
MISO/A6 D6~ 1-+PA6 PA5+-20 D5 SCK/A5
MOSI/A8/TX4 D7 2-+PA7=PB0 PA4+-19 D4~ CS/A4
A9/RX4 D8 3-+PB1 PC19+-18 D17 SWCLK
/RST D9 4-+PB7 PA3+-17 D3~ RX2/A3*
USBDM D10 5-+PC16=PC11 PA2+-16 D2~ TX2/A2
USBDP D11 6-+PC17=PC10 PA1+-15 D1~ A1
GND 7-+VSS PA0+-14 D0~ A0
SWDIO D16 8-+PC18 PC3+-13 D15~ A13
VCC 9-+VDD PA10+-12 D14 SCL/TX1
D12 10-+PA9 PA11+-11 D13 SDA/RX1
+-------------+
*A3, VREF and hardware I2C don't work on CH32X033F8P6 0-series (lot number with the penultimate bit 5 being 0).

 
Tested features

digitalWrite()/digitalRead() - can use Arduino pin numbers or PAx notation.
analogWrite() - 12-bit resolution, pins marked with ~ in pinout above.
analogRead() - very stable 12-bit resolution, but issues with A3/A0/PADC_VREF
Serial.print() - Tested 115200 bps on Serial2 (PA2/PA3) and Serial1 (PA10/PA11), as set in variant_CH32X033F8P6.h
EEPROM library - may need improvement (updated to support 122 bytes).
Pin PB7 (Arduino pin 9) can be configured as hardware reset /RST using WCH Link Utility. (See issue Hardware reset pin doesn't work on CH32X033 or CH32X035 #123)
Known issues/limitations

Pins PA7/PB0, PC16/PC11 and PC17/PC10 cannot be used for output.
Any signal on A0 seems to show on other ADC pins when disconnected.
A3, PADC_VREF and I2C don't work on CH32X033F8P6 0-series (lot number with the penultimate bit 5 being 0).
To counter missing I2C the Software_I2C library by Seeed Studio is a good alternative, although it needs some changes to improve compatibility in I2C scanning.

openwch/arduino_core_ch32#171

Author: jobitjoseph

variants/CH32X035/CH32X033F8P/README.md

@@ -0,0 +1,54 @@
+## CH32X033F8P6 ##
+
+### CH32X035/X033 Main Features ###
+-	48MHz RC oscillator
+-	20KB SRAM
+-	62KB Flash + 3328B System Flash
+-	256B system config + 256B user storage
+-	2x OPA, 3x CMP, 12-bit 14-chn ADC, 14-chn touch
+-	3x 16-bit timer, 2x WD timer, systick
+-	4x USART, I2C, SPI, USB2, USB PD, 2-wire debug
+
+
+### TSSOP20 Pinout ###
+- See [datasheet](https://www.wch.cn/downloads/CH32X035DS0_PDF.html) for complete pin function list.
+```
+                  +------v------+
+    MISO/A6 D6~ 1-+PA6       PA5+-20  D5 SCK/A5
+MOSI/A8/TX4 D7  2-+PA7=PB0   PA4+-19  D4~ CS/A4
+     A9/RX4 D8  3-+PB1      PC19+-18  D17 SWCLK
+       /RST D9  4-+PB7       PA3+-17  D3~ RX2/A3*
+     USBDM D10  5-+PC16=PC11 PA2+-16  D2~ TX2/A2
+     USBDP D11  6-+PC17=PC10 PA1+-15  D1~ A1
+           GND  7-+VSS       PA0+-14  D0~ A0
+     SWDIO D16  8-+PC18      PC3+-13  D15~ A13
+           VCC  9-+VDD      PA10+-12  D14/SCL
+           D12 10-+PA9      PA11+-11  D13/SDA
+                  +-------------+
+*A3 and I2C don't work on CH32X033F8P6 0-series (lot number with the penultimate bit 5 being 0).
+```
+
+
+### Tested features ###
+- digitalWrite()/digitalRead() - can use Arduino pin numbers or PAx notation.
+- analogWrite() - 12-bit resolution, pins marked with ~ in pinout above.
+- analogRead() - very stable 12-bit resolution, but issues with A3/A0/PADC_VREF
+- Serial.print() - Tested 115200 bps on PA2/PA3 (as set in variant_CH32X033F8P6.h)
+- EEPROM library - may need improvement (updated to support 122 bytes).
+- Pin PB7 (Arduino pin 9) can be configured as hardware reset /RST using WCH Link Utility. (See issue #123)
+
+### Known issues/limitations ###
+- Pins PA7/PB0, PC16/PC11 and PC17/PC10 cannot be used for output.
+- Any signal on A0 seems to show on other ADC pins when disconnected.
+- A3, PADC_VREF and I2C don't work on CH32X033F8P6 0-series (lot number with the penultimate bit 5 being 0).
+- To counter missing I2C the [Software_I2C library](https://github.com/Seeed-Studio/Arduino_Software_I2C) by Seeed Studio is a good alternative, although it needs some changes to improve compatibility in I2C scanning.
+
+### References ###
+- [datasheet](https://www.wch.cn/downloads/CH32X035DS0_PDF.html)
+- [reference manual](https://www.wch.cn/downloads/CH32X035RM_PDF.html)
+- [X033/X035 ](https://github.com/openwch/ch32x035),
+- [WCH examples](https://github.com/openwch/ch32x035/blob/main/EVT/EXAM)
+- [PIOC](https://github.com/openwch/ch32x035/tree/main/EVT/EXAM/PIOC)
+- [User manual](https://github.com/openwch/ch32x035/blob/main/EVT/EXAM/PIOC/PIOC%20UserManual.pdf),
+- [Reference manual](https://github.com/openwch/ch32x035/blob/main/EVT/EXAM/PIOC/PIOC-EN.pdf),
+- [Instruction set](https://github.com/openwch/ch32x035/blob/main/EVT/EXAM/PIOC/CHRISC8B-EN.pdf)