mb/google/hatch: Preserve MRC training data across FW update

Add PRESERVE to UNIFIED_MRC_CACHE so that we don't retain the memory
training data upon a FW update unless we need to.  We have had users
complaining that a 15 second memory training upon update makes them
believe that their device is not booting, thus many of them hard
resetting before bootup.

BUG=b:142084637
BRANCH=None
TEST=flash RW_SECTION_A, RW_SECTION_B, and WP_RO sections and make
     sure memory training doesn't occur on following bootup.

Change-Id: Ia5eb228b1f665a8371982544723dab3dfc40d401
Signed-off-by: Shelley Chen <shchen@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/35803
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>
Reviewed-by: Paul Fagerburg <pfagerburg@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/google/hatch/chromeos.fmd

@@ -19,7 +19,7 @@ FLASH@0xfe000000 0x2000000 {
 			RW_FWID_B@0x3dffc0 0x40
 		}
 		RW_MISC@0x17c0000 0x40000 {
-			UNIFIED_MRC_CACHE@0x0 0x30000 {
+			UNIFIED_MRC_CACHE(PRESERVE)@0x0 0x30000 {
 				RECOVERY_MRC_CACHE@0x0 0x10000
 				RW_MRC_CACHE@0x10000 0x20000
 			}