iio: proximity: vcnl3020: add proximity rate

Add the proximity rate optional option and handling of it for
vishay vcnl3020.

Signed-off-by: Ivan Mikhaylov <i.mikhaylov@yadro.com>
Link: https://lore.kernel.org/r/20210225201444.12983-2-i.mikhaylov@yadro.com


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