board: cssi: Add support for SPI bus on MCR3000 board

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>