dt-bindings: iio: light: bh1750: Add device tree binding documentation

Document device tree bindings for ROHM BH1750 ambient light sensor driver.
Signed-off-by: ryang <decatf@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/light/bh1750.txt

+ROHM BH1750 - ALS, Ambient light sensor
+
+Required properties:
+
+- compatible: Must be one of:
+    "rohm,bh1710"
+    "rohm,bh1715"
+    "rohm,bh1721"
+    "rohm,bh1750"
+    "rohm,bh1751"
+- reg: the I2C address of the sensor
+
+Example:
+
+light-sensor@23 {
+	compatible = "rohm,bh1750";
+	reg = <0x23>;
+};