dt-bindings:iio:dac:adi,ad7303 yaml conversion

Converted to maintain the requirement for Vdd-supply as per original file.
It is possible we could relax this requirement to make it at least one
of Vdd-supply and REF-supply.  We need to establish the scaling of the
output channel and if REF-supply is provided that is used instead of
Vdd-supply, hence I cannot see why a dummy regulator cannot be used for
Vdd-supply if this happens.

For now, let us keep it simple.

Drop adi,use-external-reference from binding example as no such binding
exists.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Link: https://lore.kernel.org/r/20201031134110.724233-18-jic23@kernel.org