dts: arm: st: add pwr peripheral & wake-up pins nodes

Add devicetree node of stm32 PWR peripheral that controlls wake-up pins.
The new node includes child nodes for wake-up pins configuration.
We only add these nodes for STM32 SoC series that support Poweroff.

Signed-off-by: Abderrahmane Jarmouni <abderrahmane.jarmouni-ext@st.com>