iio: light: Add driver for Silabs si1132, si1141/2/3 and si1145/6/7 ambient light, uv index and proximity sensors

The si114x supports x=1,2,3 IR LEDs for proximity sensing together with
visible and IR ambient light sensing (ALS).

Newer parts (si1132, si1145/6/7) can measure UV light and compute an UV
index
This was tested on si1143 and si1145
Signed-off-by: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Signed-off-by: Crestez Dan Leonard <leonard.crestez@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>