intel car: Use MTRR WRPROT type for XIP cache

XIP cachelines contain the executable to run, we never want
that to get modified. With the change such erronous writes
are ignored and next cacheline miss will fetch from boot
media (SPI / FWH flash).

Change-Id: I52b62866b5658e103281ffa1a91e1c64262f3175
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/15778
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/cpu/intel/car/cache_as_ram_ht.inc

@@ -310,7 +310,7 @@ no_msr_11e:
 	 */
 	movl	$copy_and_run, %eax
 	andl	$(~(CONFIG_XIP_ROM_SIZE - 1)), %eax
-	orl	$MTRR_TYPE_WRBACK, %eax
+	orl	$MTRR_TYPE_WRPROT, %eax
 	wrmsr
 
 	movl	$MTRR_PHYS_MASK(1), %ecx

src/cpu/intel/model_6ex/cache_as_ram.inc

@@ -111,7 +111,7 @@ clear_mtrrs:
 	 */
 	movl	$copy_and_run, %eax
 	andl	$(~(CONFIG_XIP_ROM_SIZE - 1)), %eax
-	orl	$MTRR_TYPE_WRBACK, %eax
+	orl	$MTRR_TYPE_WRPROT, %eax
 	wrmsr
 
 	movl	$MTRR_PHYS_MASK(1), %ecx