iio: cros_ec_light_prox: add ChromeOS EC Light and Proximity Sensors

Handle Light and Proximity sensors presented by the ChromeOS EC Sensor hub.
Creates an IIO device for each functions.

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Signed-off-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>