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>
diff --git a/src/northbridge/intel/nehalem/gma.c b/src/northbridge/intel/nehalem/gma.c
index 039923c..be17e2f 100644
--- a/src/northbridge/intel/nehalem/gma.c
+++ b/src/northbridge/intel/nehalem/gma.c
@@ -15,6 +15,7 @@
  */
 
 #include <arch/io.h>
+#include <device/mmio.h>
 #include <console/console.h>
 #include <delay.h>
 #include <device/device.h>