dt-bindings: iio: imx8qxp-adc: add missing vref-supply

Although this property is used right now for IIO_CHAN_INFO_SCALE,
this ADC has two internal reference voltages, which the driver currently
doesn't make use of.

Fixes: db73419d ("dt-bindings: iio: adc: Add binding documentation for NXP IMX8QXP ADC")
Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Reviewed-by: Haibo Chen <haibo.chen@nxp.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230424092312.61746-1-alexander.stein@ew.tq-group.com


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