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/soc/samsung/exynos5420/cpu.c b/src/soc/samsung/exynos5420/cpu.c
index fa4cd06..1c354e6 100644
--- a/src/soc/samsung/exynos5420/cpu.c
+++ b/src/soc/samsung/exynos5420/cpu.c
@@ -16,6 +16,7 @@
 
 #include <arch/cache.h>
 #include <console/console.h>
+#include <device/mmio.h>
 #include <delay.h>
 #include <device/device.h>
 #include <ec/google/chromeec/ec.h>