iio: ltr501: Add support for ltr301 chip

Added support for Liteon 301 Ambient light sensor. Since
LTR-301 and LTR-501 are register compatible(and even have same
part id), LTR-501 driver has been extended to support both
devices. LTR-501 is similar to LTR-301 in ALS sensing, But the
only difference is, LTR-501 also supports proximity sensing.

LTR-501 - ALS + Proximity combo
LTR-301 - ALS sensor.

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