ARM: dts: stm32: enable ADC on stm32h743i-eval board

There's a potentiometer connected to ADC1 and ADC2 in0 on
stm32h743i-eval board.
- Add fixed-voltage 'vdda' regulator that supplies 'vref' pin.
  It's used as voltage reference for ADC and/or DAC.
- Enable ADC1 in0 input (arbitrary choice: could be ADC2 as well).
Note: No pinctrl is needed to use in0 dedicated analog input pin
(e.g. ADC12_INP0).

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