iio: light: DT binding docs for APDS9960 driver

Document compatible string, and required DT properties for APDS9960
chipset driver.
Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Matt Ranostay <mranostay@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

Documentation/devicetree/bindings/iio/light/apds9960.txt

+* Avago APDS9960 gesture/RGB/ALS/proximity sensor
+
+http://www.avagotech.com/docs/AV02-4191EN
+
+Required properties:
+
+  - compatible: must be "avago,apds9960"
+  - reg: the I2c address of the sensor
+  - 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.
+
+Example:
+
+apds9960@39 {
+	compatible = "avago,apds9960";
+	reg = <0x39>;
+	interrupt-parent = <&gpio1>;
+	interrupts = <16 1>;
+};