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

Datasheet:
http://datasheets.maximintegrated.com/en/ds/MAX5481-MAX5484.pdfSigned-off-by: NMaury Anderson <maury.anderson@rockwellcollins.com>
Signed-off-by: NMatthew Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: NSlawomir Stepien <sst@poczta.fm>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NJonathan Cameron <jic23@kernel.org>