samples: boards: stm32 serial wakeup form LPUART on nucleo_wl55jc

This overlay shows the LPUART at 9600 with wakeUp capability
to exit the nucleo_wl55jc target plaform from the low Power
stop mode, EXTI_Line 28 is the corresponding wakeUp source.
The LPUART is sourced by the LSE (32768Hz) and max baudrate
is 9600. Set the console to 9600.

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