arch/x86: Add <arch/romstage.h>

Start with moving all postcar_frame related function
declarations here from <arch/cpu.h>.

Change-Id: I9aeef07f9009e44cc08927c85fe1862edf5c70dc
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/34911
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: HAOUAS Elyes <ehaouas@noos.fr>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/northbridge/intel/x4x/memmap.c

@@ -20,7 +20,7 @@
 #include <cbmem.h>
 #include <commonlib/helpers.h>
 #include <stdint.h>
-#include <arch/cpu.h>
+#include <arch/romstage.h>
 #include <device/pci_ops.h>
 #include <device/pci_def.h>
 #include <console/console.h>