mfd: arizona: Include linux/of.h header

'of_match_ptr' is defined in linux/of.h. Include it explicitly to
avoid breakage in the future.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
diff --git a/drivers/mfd/arizona-i2c.c b/drivers/mfd/arizona-i2c.c
index 51dbabf..beccb79 100644
--- a/drivers/mfd/arizona-i2c.c
+++ b/drivers/mfd/arizona-i2c.c
@@ -17,6 +17,7 @@
 #include <linux/regmap.h>
 #include <linux/regulator/consumer.h>
 #include <linux/slab.h>
+#include <linux/of.h>
 
 #include <linux/mfd/arizona/core.h>