nb/intel/i440bx: Make ROM area unavailable for MMIO

Change-Id: Iede1452cce8a15f85d70a3c38b4ec9e2d4a54f9e
Signed-off-by: Keith Hui <buurin@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/41091
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/northbridge/intel/i440bx/acpi/sb_pci0_crs.asl

@@ -62,6 +62,7 @@ Method(_CRS, 0) {
 	 */
 	MM1B = \_SB.PCI0.NB.TOM1
 	Local0 = 0x10000000 << 4
+	Local0 -= CONFIG_ROM_SIZE
 	MM1L = Local0 - MM1B
 
 	Return(TMP)