drivers:iio:light:isl29125: added macros for sensing range

Added macros for sensing range as the corresponding magic numbers
were used at multiple places.
   - ISL29125_SENSING_RANGE_0 for 375 lux full range
   - ISL29125_SENSING_RANGE_1 for 10k lux full range

Signed-off-by: Bijosh Thykkoottathil <bijosh.t@hotmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>