boards: arm: stm32g071 nucleo defines its vref for ADC
boards: arm: stm32g071 nucleo defines its vref for ADC

The Vref is defined by the nucleo board hardware.
This property is common to ADC channels
and required in the DTS.

Signed-off-by: Francois Ramu <b83adc7f8a23ee911a37fcb61c4033581aae0b7a@st.com>

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