iio: vcnl4000: Add event support for VCNL4010/20.

The VCNL4010 and VCNL4020 chips are able to raise interrupts on proximity
threshold events. Add support for threshold rising and falling events for
those two chips.
Signed-off-by: Mathieu Othacehe <m.othacehe@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>