iio: ltr501: Add light channel support

Added support to calculate lux value from visible
and IR spectrum adc count values. Also added IIO_LIGHT
channel to enable user read the lux value directly
from device using illuminance input ABI.

Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>