boards: nucleo_wl55jc: assert SRST before any target connection

Failed to flash blinky example to stock nucleo_wl55jc due to default
application entering low power mode using WFI. To connect to a sleeping
stm32wl55 a system reset is required.

Signed-off-by: Jelle De Vleeschouwer <jelledevleeschouwer@gmail.com>