ARM: dts: stm32: add adc support to stm32mp13

Add ADC1 and ADC2 support to STM32MP13 SoC family.

The STM32MP131 provides only ADC2, while other STM32MP13 SoCs provide
both ADC1 and ADC2.

Internal channels support limitations:
- VREFINT internal channel requires calibration data from OTP memory.
  The nvmem properties used to access OTP are not defined for time being,
  as OTP support is not yet enabled.

- VBAT internal channel is not defined by default in SoC DT, and
  has be defined in board DT when needed, instead. This avoids unwanted
  current consumption on battery, when ADC conversions are performed
  on any other channels.

Signed-off-by: Olivier Moysan <olivier.moysan@foss.st.com>
Reviewed-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>