device/mmio.h: Add include file for MMIO ops

MMIO operations are arch-agnostic so the include
path should not be arch/.

Change-Id: I0fd70f5aeca02e98e96b980c3aca0819f5c44b98
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/31691
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/drivers/intel/fsp2_0/util.c

@@ -11,7 +11,7 @@
  * (at your option) any later version.
  */
 
-#include <arch/io.h>
+#include <device/mmio.h>
 #include <cf9_reset.h>
 #include <console/console.h>
 #include <fsp/util.h>