Add the Raspberry Pi SPI controller driver.

Reviewed by:	rpaulo
Approved by:	adrian (mentor)
Add an OFW SPI compatible bus.  Fix the spibus probe to return
BUS_PROBE_GENERIC and not BUS_PROBE_SPECIFIC (0) so the OFW SPI bus can
attach when enabled.  Export the spibus devclass_t and driver_t
declarations.

Submitted by:	ray
Approved by:	adrian (mentor)