iio: add APDS9300 ambilent light sensor driver

This patch adds IIO driver for APDS9300 ambient light sensor (ALS).
http://www.avagotech.com/docs/AV02-1077EN

The driver allows to read raw data from ADC registers or calculate
lux value. It also can handle threshold interrupt.

Signed-off-by: Oleksandr Kravchenko <o.v.kravchenko@globallogic.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

Documentation/devicetree/bindings/iio/light/apds9300.txt

@@ -0,0 +1,22 @@
+* Avago APDS9300 ambient light sensor
+
+http://www.avagotech.com/docs/AV02-1077EN
+
+Required properties:
+
+  - compatible : should be "avago,apds9300"
+  - reg : the I2C address of the sensor
+
+Optional properties:
+
+  - interrupt-parent : should be the phandle for the interrupt controller
+  - interrupts : interrupt mapping for GPIO IRQ
+
+Example:
+
+apds9300@39 {
+	compatible = "avago,apds9300";
+	reg = <0x39>;
+	interrupt-parent = <&gpio2>;
+	interrupts = <29 8>;
+};