soc/intel/cannonlake: Disable RTC write protect

The cannonlake FSP enables PchLockDownRtcMemoryLock by default, but we
need this memory to be writable.  We normally over-ride this in the
SoC chip init code, so we'll do the same on cannonlake.

BUG=b:71722386
BRANCH=none
TEST=Filled /dev/nvram with 0xff and 0x00 bytes to verify we can flip
all the bits.

Change-Id: I7cdd4abc2b3795d7dd82236fbe3c112428ee882b
Signed-off-by: Caveh Jalali <caveh@chromium.org>
Reviewed-on: https://review.coreboot.org/25069
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Vincent Palatin <vpalatin@chromium.org>

src/soc/intel/cannonlake/chip.c

@@ -200,6 +200,9 @@ void platform_fsp_silicon_init_params_cb(FSPS_UPD *supd)
 
 	mainboard_silicon_init_params(params);
 
+	/* Unlock upper 8 bytes of RTC RAM */
+	params->PchLockDownRtcMemoryLock = 0;
+
 	/* SATA */
 	params->SataEnable = config->SataEnable;
 	params->SataMode = config->SataMode;