dt-bindings:iio:dac:ad5592r: txt to yaml format conversion.

This is a more complex binding. Whilst conversion is straight forward
I am unsure if the full nature of required properties has been captured.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Michael Hennerich <michael.hennerich@analog.com>
Link: https://lore.kernel.org/r/20201031184854.745828-39-jic23@kernel.org