mb/google/rex: Create 64MB AP Firmware binary for Proto 0

This patch provides a mitigation path for having different size SPINOR
parts across Rex board revisions. Rex Proto 0 only has 64MB SPINOR
mounted on the board, and the plan is to use 32MB later with Proto 1
onwards.

Hence, the idea here is to maintain a 32MB SPI Flash layout across all
Rex board revisions, but the Proto 0 build only selects
BOARD_ROMSIZE_KB_65536 config for adding padding at the end of the
32MB range.

BUG=b:242825380
TEST=Able to create 64MB AP Firmware for Rex with below layout:
SI_ALL: 0-9MB
SI_BIOS: 9MB-32MB
Padding/Unused: 32MB-64MB

Additionally, able to hit CPU reset on MTLRVP (has 64MB SPINOR) with
Rex AP Firmware binary.

Signed-off-by: Subrata Banik <subratabanik@google.com>
Change-Id: Ibcc2206456639ef4ff22e0c4069521e583be58cd
Reviewed-on: https://review.coreboot.org/c/coreboot/+/66828
Reviewed-by: Tarun Tuli <taruntuli@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Kapil Porwal <kapilporwal@google.com>
Reviewed-by: Ivy Jian <ivy.jian@quanta.corp-partner.google.com>
Reviewed-by: Eric Lai <eric_lai@quanta.corp-partner.google.com>
diff --git a/src/mainboard/google/rex/Kconfig b/src/mainboard/google/rex/Kconfig
index 90c4c2c..fdcf8bb 100644
--- a/src/mainboard/google/rex/Kconfig
+++ b/src/mainboard/google/rex/Kconfig
@@ -1,6 +1,8 @@
 config BOARD_GOOGLE_REX_COMMON
 	def_bool n
-	select BOARD_ROMSIZE_KB_32768
+	# FIXME: Rex Proto 0 only has 64MB SPINOR mounted on the board,
+	# and the plan is to use 32MB later with Proto 1 onwards.
+	select BOARD_ROMSIZE_KB_65536
 	select EC_GOOGLE_CHROMEEC_BOARDID
 	select HAVE_ACPI_RESUME
 	select HAVE_ACPI_TABLES