iio: max5487: Add support for Maxim digital potentiometers

Add implementation for Maxim MAX5487, MAX5488, MAX5489
digital potentiometers.

Datasheet:
http://datasheets.maximintegrated.com/en/ds/MAX5487-MAX5489.pdf



Signed-off-by: Cristina Moraru <cristina.moraru09@gmail.com>
CC: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>