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/northbridge/intel/e7505/memmap.c

@@ -54,6 +54,8 @@ void fill_postcar_frame(struct postcar_frame *pcf)
 	 * operations when source is left as UC.
 	 */
 
+	pcf->skip_common_mtrr = 1;
+
 	/* Cache RAM as WB from 0 -> CACHE_TMP_RAMTOP. */
 	postcar_frame_add_mtrr(pcf, 0, CACHE_TMP_RAMTOP, MTRR_TYPE_WRBACK);