iio:ak8975 Add support for gpios DT property

Add support for parsing 'gpios' property when initializing
from oftree.
This patch adds also the binding documentation file.
Signed-off-by: NJacek Anaszewski <j.anaszewski@samsung.com>
Signed-off-by: NKyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: NJonathan Cameron <jic23@kernel.org>

Documentation/devicetree/bindings/iio/magnetometer/ak8975.txt

+* AsahiKASEI AK8975 magnetometer sensor
+
+Required properties:
+
+  - compatible : should be "asahi-kasei,ak8975"
+  - reg : the I2C address of the magnetometer
+
+Optional properties:
+
+  - gpios : should be device tree identifier of the magnetometer DRDY pin
+
+Example:
+
+ak8975@0c {
+        compatible = "asahi-kasei,ak8975";
+        reg = <0x0c>;
+        gpios = <&gpj0 7 0>;
+};

drivers/iio/magnetometer/ak8975.c

@@ -29,6 +29,7 @@
 #include <linux/delay.h>
 
 #include <linux/gpio.h>
+#include <linux/of_gpio.h>
 
 #include <linux/iio/iio.h>
 #include <linux/iio/sysfs.h>
@@ -384,10 +385,15 @@ static int ak8975_probe(struct i2c_client *client,
 	int err;
 
 	/* Grab and set up the supplied GPIO. */
-	if (client->dev.platform_data == NULL)
-		eoc_gpio = -1;
-	else
+	if (client->dev.platform_data)
 		eoc_gpio = *(int *)(client->dev.platform_data);
+	else if (client->dev.of_node)
+		eoc_gpio = of_get_gpio(client->dev.of_node, 0);
+	else
+		eoc_gpio = -1;
+
+	if (eoc_gpio == -EPROBE_DEFER)
+		return -EPROBE_DEFER;
 
 	/* We may not have a GPIO based IRQ to scan, that is fine, we will
 	   poll if so */