dt-bindings:iio:dac:dpot-dac: yaml conversion.

Txt to yaml format conversion.  I dropped the example section
describing the measurement ADC, as that isn't strictly part
of this binding.

Uses the new dt-schema/schema/iio/iio-consumer.yaml schema.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Peter Rosin <peda@axentia.se>
Link: https://lore.kernel.org/r/20201031181242.742301-3-jic23@kernel.org