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

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

Add read/write attribute for proximity and illuminance period. The
period is set in the interrupt persistence flags(PS_PERS and ALS_PERS).
An interrupt will not be asserted if the raw value is not over (or lower)
than the threshold for the set continued amount of measurements.
The time in seconds is calculated by the number of continued refreshes
multiplied with the integration time.
It will always pick the next lower possible value. The period changes,
if the integration time is changed.

Signed-off-by: Astrid Rost <astrid.rost@axis.com>
Link: https://lore.kernel.org/r/20230613135025.2596641-7-astrid.rost@axis.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>