iio: max5481: Add support for Maxim digital potentiometers

Add implementation for Maxim Integrated 5481, 5482, 5483,
and 5484 digital potentiometer devices.

Datasheet:
http://datasheets.maximintegrated.com/en/ds/MAX5481-MAX5484.pdf



Signed-off-by: Maury Anderson <maury.anderson@rockwellcollins.com>
Signed-off-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Slawomir Stepien <sst@poczta.fm>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>