iio: light: vcnl4000: Add als_it for vcnl4040/4200

Add illuminance integration time for vcnl4040 and vcnl4200.
Add read/write attribute for illuminance integration time and read
attribute for available integration times.
Set scale and sampling rate according to the integration time.

Signed-off-by: Astrid Rost <astrid.rost@axis.com>
Link: https://lore.kernel.org/r/20230613135025.2596641-5-astrid.rost@axis.com


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