iio: da9150: Add DT binding documentation for GPADC

Signed-off-by: NAdam Thomson <Adam.Thomson.Opensource@diasemi.com>
Acked-by: NJonathan Cameron <jic23@kernel.org>
Signed-off-by: NSebastian Reichel <sre@kernel.org>

Documentation/devicetree/bindings/iio/adc/da9150-gpadc.txt

+Dialog Semiconductor DA9150 IIO GPADC bindings
+
+Required properties:
+- compatible: "dlg,da9150-gpadc" for DA9150 IIO GPADC
+- #io-channel-cells: Should be set to <1>
+  (See Documentation/devicetree/bindings/iio/iio-bindings.txt for further info)
+
+For further information on GPADC channels, see device datasheet.
+
+
+Example:
+
+	gpadc: da9150-gpadc {
+		compatible = "dlg,da9150-gpadc";
+		#io-channel-cells = <1>;
+	};