iio: proximity: Add rfd77402 driver

Driver for RF Digital RFD77402 VCSEL (vertical-cavity surface-emitting
laser) Time-of-Flight (ToF) sensor to measure distance up to 2 m with
millimeter precision

Signed-off-by: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>