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/soc/intel/skylake/pmutil.c

@@ -20,7 +20,7 @@
  */
 
 #include <arch/acpi.h>
-#include <arch/io.h>
+#include <device/mmio.h>
 #include <device/pci_ops.h>
 #include <device/device.h>
 #include <device/pci.h>