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

Implement the hook for sys_poweroff based on the SOFT_OFF code. Note
that standby mode was a substate of SOFT_OFF, however, it was not
supported judging from defined DT states. It can be added later using
the STANDBY state.

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