sb/intel/common/spi.c: Add a SPI write protect function
Could be useful to write protect regions like for instance the
MRC_CACHE region.
Tested on Intel DG41WV (i82801gx) and Lenovo Thinkpad X220 (bd82x6x)
to write protect the mrc_cache region.
Change-Id: Id0a9a0de639c5d6761a77a56ceba6d89110a4ea1
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/23146
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>