iio: light: vcnl4000: Add ps_it attributes for vcnl4040

Add read/write attribute for proximity integration time, and read
attribute for available proximity integration times for the vcnl4040
chip.

Signed-off-by: Mårten Lindahl <marten.lindahl@axis.com>
Link: https://lore.kernel.org/r/20220926091900.1724105-3-marten.lindahl@axis.com


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