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/drivers/amd/agesa/romstage.c

@@ -107,3 +107,9 @@ asmlinkage void car_stage_entry(void)
 {
 	romstage_main();
 }
+
+void *cbmem_top_chipset(void)
+{
+	/* Top of CBMEM is at highest usable DRAM address below 4GiB. */
+	return (void *)restore_top_of_low_cacheable();
+}