reset: ath79: add missing include

The driver uses readl/writel, so it should include linux/io.h.
Acked-by: Aban Bedel <albeu@free.fr>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>

drivers/reset/reset-ath79.c

@@ -12,6 +12,7 @@
  * GNU General Public License for more details.
  */
 
+#include <linux/io.h>
 #include <linux/module.h>
 #include <linux/platform_device.h>
 #include <linux/reset-controller.h>