dt-bindings:iio:dac:ti,dac5571 yaml conversion.

A few tweaks in this conversion.
* The example didn't have the I2C address of 4C in the node name so
  fixed that.
* The reference voltage in the txt file is an optional binding, but
  the driver is making use of it to provide the scaling of the output
  channels.  As such I have made it required going forwards.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Sean Nyekjaer <sean@geanix.com>
Link: https://lore.kernel.org/r/20201031134110.724233-13-jic23@kernel.org