arch/x86: Add postcar_frame_common_mtrrs()

As most platforms will share the subset of enabling
both low RAM WB and high ROM WP MTRRs, provide them
with a single function.

Add possibility for the platform to skip these if
required.

Change-Id: Id1f8b7682035e654231f6133a42909a36e3e15a1
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/34809
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/cpu/intel/car/romstage.c

@@ -33,6 +33,8 @@ static void prepare_and_run_postcar(struct postcar_frame *pcf)
 
 	fill_postcar_frame(pcf);
 
+	postcar_frame_common_mtrrs(pcf);
+
 	run_postcar_phase(pcf);
 	/* We do not return here. */
 }