STM32: RTC: Call irq_handler whenever interrupt fires
STM32: RTC: Call irq_handler whenever interrupt fires

lp_ticker driver is the known registered user of RTC handler API.

In case, a lp_ticker is set in the past, the lp_ticker_fire_interrupt
will be called which itself sets the RTC interrupt as pending by calling
NVIC_SetPendingIRQ(RTC_WKUP_IRQn). This all happens without actual
programing of the RTC wake-up.

As a result the RTC HW and corresponding HAL layer doesn't expect an
interrupt to happen and will not call HAL_RTCEx_WakeUpTimerEventCallback.

To sove this situation, we will not use HAL_RTCEx_WakeUpTimerEventCallback
weak definition but rather call handler whenever an RTC interrupt fires.

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