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>
diff --git a/src/arch/x86/Makefile.inc b/src/arch/x86/Makefile.inc
index 5f184c5..04e679f 100644
--- a/src/arch/x86/Makefile.inc
+++ b/src/arch/x86/Makefile.inc
@@ -280,6 +280,7 @@
 
 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