iio: light: ltr390: Add configurable gain and resolution

Add support for configuring and reading the gain and resolution
(integration time). Also provide the available values for gain and
resoltion respectively via `read_avail` callback.

Signed-off-by: Abhash Jha <abhashkumarjha123@gmail.com>
Link: https://patch.msgid.link/20240731063706.25412-2-abhashkumarjha123@gmail.com


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