dt-bindings: iio: light: add AMS AS73211 support

Add DT bindings for AMS AS73211 XYZ True Color Sensor.

Signed-off-by: Christian Eggers <ceggers@arri.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>