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. Sending any-
thing on SPI_CSLOW and SPI_CSHIGH is expected to select/deselect
the device, and writing to SPI_DATA is expected to initiate a
byte exchange. The result of the exchange is excpected to be re-
trieved by reading SPI_DATA.

Provides the SPI relay protocol.

Load driver with "596 LOAD".