iio: light: add APDS9960 ALS + promixity driver

APDS9960 is a combination of ALS, proximity, and gesture sensors.

This patch adds support for these functions along with gain control,
integration time, and event thresholds.

Signed-off-by: Matt Ranostay <mranostay@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>