iio: light: add support for TI's opt3001 light sensor

TI's opt3001 light sensor is a simple and yet powerful
little device. The device provides 99% IR rejection,
automatic full-scale, very low power consumption and
measurements from 0.01 to 83k lux.

This patch adds support for that device using the IIO
framework.

See http://www.ti.com/product/opt3001 for more information.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Andreas Dannenberg <dannenberg@ti.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>