dt-bindings: iio: adc: stm32: add missing vdda supply

Add missing vdda-supply, analog power supply, to STM32 ADC. It's required
to properly supply the ADC.

Fixes: 841fcea4 ("Documentation: dt-bindings: Document STM32 ADC DT
bindings")
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/adc/st,stm32-adc.txt

@@ -38,6 +38,7 @@ Required properties:
     It's required on stm32h7.
 - clock-names: Must be "adc" and/or "bus" depending on part used.
 - interrupt-controller: Identifies the controller node as interrupt-parent
+- vdda-supply: Phandle to the vdda input analog voltage.
 - vref-supply: Phandle to the vref input analog reference voltage.
 - #interrupt-cells = <1>;
 - #address-cells = <1>;