google/butterfly: Declare mrc_scrambler_seed_chk in cmos.layout

SandyBridge raminit uses this CMOS option. If it is not declared, the build
fails when USE_OPTION_TABLE is selected.

Change-Id: I1ba1f994d4ea3824dc66e8f35d0b5b24b88d4dd6
Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-on: http://review.coreboot.org/4269
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/mainboard/google/butterfly/cmos.layout

@@ -95,6 +95,7 @@ entries
 # SandyBridge MRC Scrambler Seed values
 896         32        r       0        mrc_scrambler_seed
 928         32        r       0        mrc_scrambler_seed_s3
+960         16        r       0        mrc_scrambler_seed_chk
 
 # coreboot config options: check sums
 984         16       h       0        check_sum