dts: bindings: power: stm32 pwr peripheral & wake-up pins

Add DT binding for stm32 PWR peripheral that controlls wake-up pins.
This binding primarily introduces wake-up pins configuration in
a unifed way that takes into consideration the variations between
STM32 SoC series & facilitates the association of GPIO pins with
their corresponding wake-up pins.

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