drivers: gpio: stm32: support wkup pins configuration

Introduce a custom STM32_GPIO_WKUP GPIO flag.
Use the newly introduced stm32_pwr_wkup_pin_cfg_gpio() public
function to configure GPIO pins, that have the STM32_GPIO_WKUP
flag in DT, as sources for STM32 PWR wake-up pins, on the condition
that there is a wake-up pin that corresponds to each of them.
These GPIO pins can then be used to power on the system after Poweroff
like a reset pin.

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