drivers: serial: stm32 uart driver set EXTI line for wakeup from lowPower

The USART or LPUART, especially on stm32wl, must set the EXTI
lines corresponding to the wakeUp source.
This LL_EXTI_LINE_xx is given by the Device Tree property.
This makes the system exiting lowpower stop mode on EXTI irq.
The LL_EXTI_LINE_x is exactly the bit(X) set.

Signed-off-by: Francois Ramu <francois.ramu@st.com>