dt-bindings: iio: dac: ad5791: Add required voltage supplies

Vcc, iovcc, vrefp, and vrefn are needed for the DAC to work.
Add them as required bindings for ad5791.

Reviewed-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Axel Haslam <ahaslam@baylibre.com>
Link: https://patch.msgid.link/20241031071746.848694-3-ahaslam@baylibre.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>