dts: binding: stm32adc set the VREf as a property

Add the Vref reference voltage in the DTS, so that the adc driver
can retrieve the value for conversion.

Signed-off-by: Francois Ramu <francois.ramu@st.com>

dts/bindings/adc/st,stm32-adc.yaml

@@ -27,6 +27,12 @@ properties:
     pinctrl-names:
       required: true
 
+    vref_mv:
+      type: int
+      required: false
+      default: 3300
+      description: Indicates the reference voltage of the ADC in mV.
+
     has-temp-channel:
       type: boolean
       description: Indicates if the ADC has a dedicated internal temperature sensor channel.