iio:dac: Add ad5755 driver

iio:dac: Add ad5755 driver

This patch adds support for the AD5755, AD5755-1, AD5757, AD5735, AD5737 16 and
14 bit quad-channel DACs. The AD5757/AD5737 only have current outputs, but
for the AD5755/AD5757 each of the outputs can be configured to either be a
voltage or a current output. We only allow to configure this at device probe
time since usually this needs to match the external circuitry and should not be
changed on the fly.

A few trivial formatting changes on merge.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>