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/pineview/memmap.c

@@ -16,7 +16,7 @@
 
 #define __SIMPLE_DEVICE__
 
-#include <arch/cpu.h>
+#include <arch/romstage.h>
 #include <device/pci_ops.h>
 #include <device/device.h>
 #include <device/pci_def.h>