mb/intel/adlrvp: Add PRESERVE to UNIFIED_MRC_CACHE

This patch preserves MRC training data across FW update.

Change-Id: I6b7273471e4fc9dc25eac80904e012f86a981836
Signed-off-by: Subrata Banik <subrata.banik@intel.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/49681
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: EricR Lai <ericr_lai@compal.corp-partner.google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/intel/adlrvp/chromeos.fmd

@@ -16,7 +16,7 @@ FLASH@0xfe000000 32M {
 			RW_FWID_B 0x40
 		}
 		RW_MISC 0x30000 {
-			UNIFIED_MRC_CACHE 0x20000 {
+			UNIFIED_MRC_CACHE(PRESERVE) 0x20000 {
 				RECOVERY_MRC_CACHE 0x10000
 				RW_MRC_CACHE 0x10000
 			}