soc: st: stm32f4: always clear wakeup flags on power off entry

STM32F4x SoCs reference manuals say that for Standby mode (aka Power OFF)
entry, "WUF bit is cleared in Power Control register (PWR_CR)" [1]
and "WUF bit is cleared in Power Control/Status register (PWR_CR)" [2].
Therefore call LL_PWR_ClearFlag_WU() unconditionally (regarding
CONFIG_STM32_WKUP_PINS) in z_sys_poweroff() sequence.

Reference manuals references:
- [1] STM32F401xx: RM0368 Rev 6, Table 20.
- [2] STM32F469xx/STM32F479xx: RM0386 Rev 6, Table 23.

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