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/drivers/amd/agesa/mtrr_fixme.c

@@ -11,7 +11,7 @@
  * GNU General Public License for more details.
  */
 
-#include <arch/cpu.h>
+#include <arch/romstage.h>
 #include <cbmem.h>
 #include <console/console.h>
 #include <commonlib/helpers.h>

src/drivers/amd/agesa/romstage.c

@@ -15,6 +15,7 @@
 
 #include <arch/acpi.h>
 #include <arch/cpu.h>
+#include <arch/romstage.h>
 #include <cbmem.h>
 #include <cpu/amd/car.h>
 #include <cpu/x86/bist.h>