dt-bindings: iio: dac: ad5686: add binding

Add a binding for AD5686

Signed-off-by: Michael Auchter <michael.auchter@ni.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200924195215.49443-3-michael.auchter@ni.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>