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>