dt-bindings: iio: light: add veml6030 ALS bindings

This commit adds device tree bindings for veml6030 ambient
light sensor.

Signed-off-by: Rishi Gupta <gupt21@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>