ARM: dts: stm32: add missing vdda-supply to adc on stm32429i-eval

Add missing vdda-supply required by STM32 ADC.

Fixes: 7465d811 ("ARM: dts: stm32: enable ADC on stm32f429i-eval
board")

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>