iio: add APDS9300 ambilent light sensor driver

This patch adds IIO driver for APDS9300 ambient light sensor (ALS).
http://www.avagotech.com/docs/AV02-1077EN



The driver allows to read raw data from ADC registers or calculate
lux value. It also can handle threshold interrupt.

Signed-off-by: Oleksandr Kravchenko <o.v.kravchenko@globallogic.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>