mb/google/bluey: Resize WP_RO and add RW_UNUSED region

Modify the flash map (fmd) to reduce the Write-Protected RO (WP_RO)
section from 12M to 8M. The recovered 4M of space is reallocated
to a new RW_UNUSED region positioned before RW_LEGACY.

BUG=None
TEST=Build bluey image and verify FMAP table with 'fmap_decode'.
Ensure resulting layout matches the intended offsets.

Change-Id: I6dde5fac5ba3be5fb28f2cd46c5518d87082c067
Signed-off-by: Subrata Banik <subratabanik@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/91379
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Kapil Porwal <kapilporwal@google.com>

src/mainboard/google/bluey/chromeos-nogsc.fmd

@@ -1,7 +1,7 @@
 ## SPDX-License-Identifier: GPL-2.0-only
 
 FLASH@0x0 CONFIG_ROM_SIZE {
-	WP_RO 12M {
+	WP_RO 8M {
 		RO_SECTION {
 			BOOTBLOCK 512K
 			FMAP 4K
@@ -37,5 +37,7 @@ FLASH@0x0 CONFIG_ROM_SIZE {
 		RW_FWID_B 256
 	}
 
+	RW_UNUSED 4M
+
 	RW_LEGACY(CBFS)
 }