soc: st: stm32c0: always clear wakeup flags on power off entry

STM32C0x SoCs reference manual says the wakeup source status flags (from
register PWR_SR1) are not cleared by hardware and therefore should be
cleared by software before entering Standby or Shutdown power mode.
Therefore call LL_PWR_ClearFlag_WU() unconditionally (regarding
CONFIG_STM32_WKUP_PINS) in z_sys_poweroff() sequence.

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