mb/google/zork: Mark RW_MRC_CACHE as "Preserve"

AGESA checks to make sure that the firmware version reading the MRC
cache is the same version that wrote it, so it doesn't need to be
erased during a firmware update.

BUG=b:173724014
TEST=Flash firmware to DUT, update firmware, check RW_MRC_CACHE was
not erased
BRANCH=Zork

Signed-off-by: Martin Roth <martin@coreboot.org>
Change-Id: Ice3d1d467c25366b7ef678cd6481d043f62644ca
Reviewed-on: https://review.coreboot.org/c/coreboot/+/47776
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Marshall Dawson <marshalldawson3rd@gmail.com>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>

src/mainboard/google/zork/chromeos.fmd

@@ -1,6 +1,6 @@
 FLASH@0xFF000000 16M {
 	SI_BIOS {
-		RW_MRC_CACHE 64K
+		RW_MRC_CACHE(PRESERVE) 64K
 		RW_SECTION_A 3M {
 			VBLOCK_A 8K
 			FW_MAIN_A(CBFS)