drivers: spi: Add support for half-duplex (3-wire) SPI

Add support for half-duplex (3-wire) SPI operation using the Raspberry
Pi Pico PIO.  To allow control of the size of the driver, including
half-duplex support is optional, under the control of Kconfig options.

The original PIO source code is also included as a reference.

Corrected 3-wire tx/rx counts.

Enable half-duplex code based on DTS configuration

Replace runtime checks with static BUILD_ASSERT()

Remove too-fussy Kconfig options

Removed PIO source per review request

Signed-off-by: Steve Boylan <stephen.boylan@beechwoods.com>