iio: light: ltr390: Add ALS channel and support for gain and resolution

Add new ALS channel and allow reading lux and scale values.
Also provide gain and resolution configuration for ALS channel.
Add automatic mode switching between the UVS and ALS channel
based on which channel is being accessed.
The default mode in which the sensor start is ALS mode.

Signed-off-by: Abhash Jha <abhashkumarjha123@gmail.com>
Link: https://patch.msgid.link/20240814113135.14575-2-abhashkumarjha123@gmail.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>