iio: gp2ap020a00f: Add a driver for the device

Add a new driver for the ambient light/proximity sensor
device. The driver exposes three channels: light_clear
light_ir and proximity. It also supports triggered buffer,
high and low ambient light threshold event and proximity
detection events.

Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>