staging: iio: add driver for isl29028

Intersil's ISL29028 is concurrent Ambient Light and
Proximity Sensor device.
Add driver to access the light and IR intensity and
proximity value via iio interface.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>