soc: st: stm32wbx: always clear wakeup flags on power off entry

STM32WBx SoCs reference manuals say that for Shutdown mode (aka Power OFF)
entry, "WUFx bits are cleared in power status register 1 (PWR_SR1)".
Therefore call LL_PWR_ClearFlag_WU() unconditionally (regarding
CONFIG_STM32_WKUP_PINS) in z_sys_poweroff() sequence.

Reference manuals references:
- STM32WB55xx/STM32WB35xx: RM0434 Rev 14, Table 34.
- STM32WB50CG/STM32WB30CE: RM0471 Rev 9, Table 32.
- STM32WB15CC: RM0473 Rev 11, Table 33.
- STM32WB10CC: RM0478 Rev 9, Table 31.

Signed-off-by: Etienne Carriere <etienne.carriere@st.com>