mb/google/guybrush,mancomb: Adjust GBB size to 12KB

The 448KB size of the GBB is larger than is needed, so adjust it down
to the minimum size needed.

BUG=b:186761897
TEST=Build & Boot guybrush

Signed-off-by: Martin Roth <martinroth@chromium.org>
Change-Id: I7a24945cd9ecc8872871f57b09ca71fc40e92473
Reviewed-on: https://review.coreboot.org/c/coreboot/+/52764
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Raul Rangel <rrangel@chromium.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>

src/mainboard/google/guybrush/chromeos.fmd

@@ -25,7 +25,7 @@ FLASH@0xFF000000 16M {
 			RO_SECTION {
 				FMAP 2K
 				RO_FRID 64
-				GBB@4K 448K
+				GBB@4K 12K
 				COREBOOT(CBFS)
 			}
 		}

src/mainboard/google/mancomb/chromeos.fmd

@@ -25,7 +25,7 @@ FLASH@0xFF000000 16M {
 			RO_SECTION {
 				FMAP 2K
 				RO_FRID 64
-				GBB@4K 448K
+				GBB@4K 12K
 				COREBOOT(CBFS)
 			}
 		}