max44000: Expose ambient sensor scaling

This patch exposes ALSTIM as illuminance_integration_time and ALSPGA as
illuminance_scale.

Changing ALSTIM also changes the number of bits available in the data
register. This is handled inside raw value reading because:
* It's very easy to shift a few bits
* It allows SCALE and INT_TIME to be completely independent controls
* Buffer support requires constant scan_type.realbits per-channel

Signed-off-by: Crestez Dan Leonard <leonard.crestez@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>