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/include/cpu/intel/romstage.h

@@ -1,7 +1,7 @@
 #ifndef _CPU_INTEL_ROMSTAGE_H
 #define _CPU_INTEL_ROMSTAGE_H
 
-#include <arch/cpu.h>
+#include <arch/romstage.h>
 
 void mainboard_romstage_entry(void);