rtc: at91sam9: include linux/of.h

This driver is using device tree but is not including of.h
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

drivers/rtc/rtc-at91sam9.c

@@ -18,6 +18,7 @@
 #include <linux/kernel.h>
 #include <linux/mfd/syscon.h>
 #include <linux/module.h>
+#include <linux/of.h>
 #include <linux/platform_data/atmel.h>
 #include <linux/platform_device.h>
 #include <linux/regmap.h>