iio: dpot-dac: DAC driver based on a digital potentiometer

It is assumed that the dpot is used as a voltage divider between the
current dpot wiper setting and the maximum resistance of the dpot. The
divided voltage is provided by a vref regulator.

.------.
.-----------. | |
| vref |--' .---.
| regulator |--. | |
'-----------' | | d |
| | p |
| | o | wiper
| | t |<---------+
| | |
| '---' dac output voltage
| |
'------+------------+

Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>