dt-bindings: iio: light: Add binding for ADUX1020

Add devicetree binding for Analog Devices ADUX1020 Photometric
sensor.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>