arch/x86: Drop CBMEM_TOP_BACKUP

Code has evolved such that there seems to be little
use for global definition of cbmem_top_chipset().
Even for AMD we had three different implementations.

Change-Id: I44805aa49eab526b940e57bd51cd1d9ae0377b4b
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/43326
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/cpu/amd/mtrr/amd_mtrr.c

@@ -1,6 +1,6 @@
 /* SPDX-License-Identifier: GPL-2.0-only */
 
-#include <cbmem.h>
+#include <amdblocks/biosram.h>
 #include <console/console.h>
 #include <device/device.h>
 #include <arch/cpu.h>