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

The VCNL4010 and VCNL4020 chips are able to raise interrupts on data ready.
Use it to provide triggered buffer support for proximity data.

Those two chips also provide ambient light data. However, they are sampled
at different rate than proximity data. As this is not handled by the IIO
framework for now, and the sample frequencies of ambient light data are
very low, do add buffer support for them.

Signed-off-by: Mathieu Othacehe <m.othacehe@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>