ARM: dts: stm32: add ADC support to stm32mp157c-ed1

ARM: dts: stm32: add ADC support to stm32mp157c-ed1

Add ADC support to stm32mp157c-ed1 board.
Following ADC signals are dedicated for analog and routed to connectors:
- ADC1/2 in0 (ANA0)
- ADC1/2 in1 (ANA1)
- ADC1 in6 (PF12)
Configure ADC1 with these signals. But keep it disabled by default, so
PF12 can be used as gpio by default.
Add VDD and VDDA supplies to ADC on stm32mp157c-ed1 board. This allows to
get full ADC analog performances in case VDDA is below 2.7V (not the case
by default).

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