spi: Add a driver for the RPI RP2040 GPIO bridge

The Raspberry Pi RP2040 GPIO bridge is an I2C-attached device exposing
both a Tx-only SPI controller, and a GPIO controller.

Due to the relative difference in transfer rates between standard-mode
I2C and SPI, the GPIO bridge makes use of 12 MiB of non-volatile storage
to cache repeated transfers. This cache is arranged in ~8 KiB blocks and
is addressed by the MD5 digest of the data contained therein.

Optionally, this driver is able to take advantage of Raspberry Pi RP1
GPIOs to achieve faster than I2C data transfer rates.

Signed-off-by: Richard Oliver <richard.oliver@raspberrypi.com>