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

Documentation/devicetree/bindings/iio/adc/nxp,imx8qxp-adc.yaml

@@ -39,6 +39,12 @@ properties:
   power-domains:
     maxItems: 1
 
+  vref-supply:
+    description: |
+      External ADC reference voltage supply on VREFH pad. If VERID[MVI] is
+      set, there are additional, internal reference voltages selectable.
+      VREFH1 is always from VREFH pad.
+
   "#io-channel-cells":
     const: 1
 
@@ -72,6 +78,7 @@ examples:
             assigned-clocks = <&clk IMX_SC_R_ADC_0>;
             assigned-clock-rates = <24000000>;
             power-domains = <&pd IMX_SC_R_ADC_0>;
+            vref-supply = <&reg_1v8>;
             #io-channel-cells = <1>;
         };
     };