soc: st: stm32l4: always clear wakeup flags on power off entry

STM32L4x 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:
- STM32L41xx/L42xx/L43xx/L44xx/L45xx/L46xx: RM0394 Rev 5, Table 29.
- STM32L47xx/L48xx/L49xx/L4Axx: RM0351 Rev 10, Table 31.

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