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 <stephen.boylan@beechwoods.com>
Steve Boylan