soc: st: stm32u5: always clear wakeup flags on power off entry

STM32U5x SoCs reference manual says that for Shutdown mode (aka Power OFF)
entry, "WUFx bits cleared in PWR_WUSR". Therefore call
LL_PWR_ClearFlag_WU() unconditionally (regarding CONFIG_STM32_WKUP_PINS)
in z_sys_poweroff() sequence.

Reference manuals references:
- STM32U5: RM0456 Rev 5, Table 107.

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