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 <stephen.boylan@beechwoods.com>