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>
