staging: iio: isl29018: document device tree bindings

Fix the following warnings from checkpatch:

WARNING: DT compatible string "isil,isl29018" appears un-documented --
check ./Documentation/devicetree/bindings/
WARNING: DT compatible string "isil,isl29023" appears un-documented --
check ./Documentation/devicetree/bindings/
WARNING: DT compatible string "isil,isl29035" appears un-documented --
check ./Documentation/devicetree/bindings/
Signed-off-by: Brian Masney <masneyb@onstation.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

Documentation/devicetree/bindings/iio/light/isl29018.txt

+* ISL 29018/29023/29035 I2C ALS, Proximity, and Infrared sensor
+
+Required properties:
+
+  - compatible: Should be one of
+		"isil,isl29018"
+		"isil,isl29023"
+		"isil,isl29035"
+  - reg: the I2C address of the device
+
+Optional properties:
+
+  - interrupt-parent: should be the phandle for the interrupt controller
+  - interrupts: the sole interrupt generated by the device
+
+  Refer to interrupt-controller/interrupts.txt for generic interrupt client
+  node bindings.
+
+  - vcc-supply: phandle to the regulator that provides power to the sensor.
+
+Example:
+
+isl29018@44 {
+	compatible = "isil,isl29018";
+	reg = <0x44>;
+	interrupt-parent = <&gpio>;
+	interrupts = <TEGRA_GPIO(Z, 2) IRQ_TYPE_LEVEL_HIGH>;
+};