dt-bindings:iio:dac:adi,ad5758 yaml conversion

I have put Michael as maintainer on this one. Happy to change it to
someone else though.

One issue in here, is I cannot have an example with a negative
limit on the range.  There are very few such yaml bindings in existence
but the thermal-zones.yaml has the same problem.  If there is
any means of fixing this let me know.  For now I'm sticking to
positive range values in the example.

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/20201031134110.724233-24-jic23@kernel.org