iio: light: Add support for ADUX1020 sensor

Add initial support for Analog Devices ADUX1020 Photometric sensor.
Only proximity mode has been enabled for now.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>