spi: constify of_device_id array

of_device_id is always used as const.
(See driver.of_match_table and open firmware functions)

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Mark Brown <broonie@kernel.org>

diff --cc drivers/spi/spi-mpc512x-psc.c
Simple merge

diff --cc drivers/spi/spi-octeon.c
Simple merge

diff --cc drivers/spi/spi-st-ssc4.c
Simple merge