SPI relay driver

This driver is designed for a ad-hoc adapter card that acts as a
SPI relay between the z80 bus and the SPI device. When writing
to SPI_CTL, we expect a bitmask of the device to select, with
0 meaning that everything is de-selected. Reading SPI_CTL
returns 0 if the device is ready or 1 if it's still running an
exchange. Writing to SPI_DATA initiates an exchange.

Provides the SPI relay protocol. Load driver with "419 LOAD".