iio: Add Vishay VEML6070 UV A light sensor driver

ultraviolet (UV) light sensor with I2C interface with a peak
sensitivity at 355 nm

strangely, chip uses two addresses 0x38 and 0x39 for LSB and
MSB data, resp.

datasheet: http://www.vishay.com/docs/84277/veml6070.pdf



Signed-off-by: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>