soc: st: stm32g0: always clear wakeup flags on power off entry

STM32G0x SoCs reference manuals say that for Shutdown mode (STM32G0x1)
or Standby mode (STM32G0x0) entry (used for Power OFF entry), that
"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:
- STM32G0x0: RM0454 Rev 5, Table 25.
- STM32G0x1: RM0444 Rev 6, Table 34.

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