soc: arm: st_stm32: wb: add support for sys_poweroff

Implement the sys_poweroff() hook. The hsem locking is not part of the
implementation, it doesn't seem necessary after
https://github.com/zephyrproject-rtos/zephyr/pull/42409

 but I may be
wrong. Needs verification.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>