iio:dac: Add DT binding documentation for ds4424

Signed-off-by: Ismail H. Kose <ihkose@gmail.com>
Signed-off-by: Ismail H. Kose <Ismail.Kose@maximintegrated.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>