soc: st: stm32wlx: always clear wakeup flags on power off entry

STM32WLx SoCs reference manual says 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:
- STM32WBLEx/STM32WL5x: RM0461 Rev 10, Table 47.

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