MCR3000 board has some components tied to the SPI bus, like the Texas
Instruments LM74 temperature sensor.
Add support for SPI bus. The SPI chipselects are a bit special in the
way that they are driven by 3 bits in a register of the board's CPLD
where the value writen in those bits exclusively activates one of the
7 possible chipselects and value 0 sets all chipselets to inactive.
So add a special GPIO driver that simulates GPIOs for those chipselect.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
