ARM: dts: stm32: add syscfg to ADC on stm32mp157c

On stm32mp157c, the ADC inputs are multiplexed with analog switches which
have reduced performances when their supply is below 2.7V (vdda by
default).
Add syscfg registers that can be used on stm32mp157c, to get full ADC
analog performances.

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