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/PinNamesVar.h

@@ -0,0 +1,11 @@
+/* Alternate pin name */
+
+/* SYS_WKUP */
+#ifdef PWR_WAKEUP_PIN1
+  SYS_WKUP1 = PA_0,
+#endif
+/* USB */
+#ifdef USBCON
+  USB_DM = PC_16,
+  USB_DP = PC_17,
+#endif