mfd: tps6586x: 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>

drivers/mfd/tps6586x.c

@@ -26,6 +26,7 @@
 #include <linux/i2c.h>
 #include <linux/platform_device.h>
 #include <linux/regmap.h>
+#include <linux/of.h>
 
 #include <linux/mfd/core.h>
 #include <linux/mfd/tps6586x.h>