dt-bindings: iio: dac: AD5766 yaml documentation

This adds device tree bindings for the AD5766 DAC.

Signed-off-by: Cristian Pop <cristian.pop@analog.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210115112105.58652-1-cristian.pop@analog.com


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