mb/google/rex: Disable stage cache

This patch disables the stage cache to save boot time.

Note: S3 is not POR for Intel MTL mobile skus.

Boot time is reduced by ~8ms.

Boot time before:
   4:end of romstage                1,391,225 (13,724)
 100:start of postcar               1,403,339 (12,114)

Boot time after:
   4:end of romstage                1,380,262 (5,618)
 100:start of postcar               1,392,323 (12,060)

Change-Id: I9775fc628f345a514894f30435a374e2ffa057c1
Signed-off-by: Subrata Banik <subratabanik@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/71695
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Tarun Tuli <taruntuli@google.com>
Reviewed-by: Kapil Porwal <kapilporwal@google.com>

src/mainboard/google/rex/Kconfig

@@ -21,6 +21,7 @@ config BOARD_GOOGLE_REX_COMMON
 	select HAVE_ACPI_TABLES
 	select I2C_TPM
 	select INTEL_LPSS_UART_FOR_CONSOLE
+	select MAINBOARD_DISABLE_STAGE_CACHE
 	select MAINBOARD_HAS_TPM2
 	select PMC_IPC_ACPI_INTERFACE
 	select SOC_INTEL_CSE_LITE_SKU