arch/x86/postcar: Support CBMEM_TOP_BACKUP

Boards with CBMEM_TOP_BACKUP=y can also use POSTCAR_STAGE
for MTRR setup after adding this file in the build.

Change-Id: I5f9a673ff59ccfbba16308d27f653f5cf3b49017
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/21445
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/arch/x86/Makefile.inc

@@ -280,6 +280,7 @@ postcar-generic-ccopts += -D__POSTCAR__
 
 postcar-y += boot.c
 postcar-y += cbfs_and_run.c
+postcar-y += cbmem.c
 postcar-y += cpu_common.c
 postcar-y += exit_car.S
 postcar-y += memcpy.c