soc/intel: Use common romstage code

This provides stack guards with checking and common
entry into postcar.

The code in cpu/intel/car/romstage.c is candidate
for becoming architectural so function prototype
is moved to <arch/romstage.h>.

Change-Id: I4c5a9789e7cf3f7f49a4a33e21dac894320a9639
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/34893
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/getac/p470/romstage.c

@@ -24,7 +24,7 @@
 #include <cpu/x86/lapic.h>
 #include <pc80/mc146818rtc.h>
 #include <console/console.h>
-#include <cpu/intel/romstage.h>
+#include <arch/romstage.h>
 #include <northbridge/intel/i945/i945.h>
 #include <northbridge/intel/i945/raminit.h>
 #include <southbridge/intel/i82801gx/i82801gx.h>