drivers: spi: spi_pico_pio: Add basic support for SPI via PIO

Add fundamental feature support for RP2040 PIO SPI peripherals.
This commit implements synchronous transfer with 8-bit MSB
format.  Using PIO allows any GPIO pins to be assigned the roles
of CS, CLK, MOSI, and MISO.

Optional features not implemented yet:

  - Interrupt based transfer
  - DMA transfer
  - Slave mode
  - Varying word size
  - 3-wire SPI support
  - LSB-first

Updated in response to review comments.
Further updates from second round of review.
Rename spi_pico_pio.c source to match zephyr/MAINTAINERS.yml
Remove unnecessary initialization code.
Resolve merge conflicts

Signed-off-by: Steve Boylan <[email protected]>

Author: ThreeEights

boards/arm/rpi_pico/doc/index.rst

@@ -92,6 +92,9 @@ hardware features:
    * - UART (PIO)
      - :kconfig:option:`CONFIG_SERIAL`
      - :dtcompatible:`raspberrypi,pico-uart-pio`
+   * - SPI (PIO)
+     - :kconfig:option:`CONFIG_SPI`
+     - :dtcompatible:`raspberrypi,pico-spi-pio`
 
 Pin Mapping
 ===========

drivers/spi/CMakeLists.txt

@@ -42,6 +42,7 @@ zephyr_library_sources_ifdef(CONFIG_SPI_SMARTBOND   spi_smartbond.c)
 zephyr_library_sources_ifdef(CONFIG_SPI_OPENTITAN	spi_opentitan.c)
 zephyr_library_sources_ifdef(CONFIG_SPI_NUMAKER		spi_numaker.c)
 zephyr_library_sources_ifdef(CONFIG_SPI_AMBIQ		spi_ambiq.c)
+zephyr_library_sources_ifdef(CONFIG_SPI_RPI_PICO_PIO	spi_rpi_pico_pio.c)
 
 zephyr_library_sources_ifdef(CONFIG_SPI_RTIO spi_rtio.c)
 zephyr_library_sources_ifdef(CONFIG_SPI_ASYNC spi_signal.c)

drivers/spi/Kconfig

@@ -131,4 +131,6 @@ source "drivers/spi/Kconfig.numaker"
 
 source "drivers/spi/Kconfig.ambiq"
 
+source "drivers/spi/Kconfig.rpi_pico"
+
 endif # SPI

drivers/spi/Kconfig.rpi_pico

@@ -0,0 +1,11 @@
+# Copyright (c) 2023 Stephen Boylan <[email protected]>
+# SPDX-License-Identifier: Apache-2.0
+
+config SPI_RPI_PICO_PIO
+	bool "Raspberry Pi PICO PIO SPI controller driver"
+	default y
+	depends on DT_HAS_RASPBERRYPI_PICO_SPI_PIO_ENABLED
+	select PICOSDK_USE_PIO
+	select PICOSDK_USE_CLAIM
+	help
+	  Enable driving SPI via PIO on the PICO