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.

targets/TARGET_STM/rtc_api.c

@@ -295,13 +295,10 @@ int rtc_isenabled(void)
 
 static void RTC_IRQHandler(void)
 {
+    /*  Update HAL state */
     HAL_RTCEx_WakeUpTimerIRQHandler(&RtcHandle);
-}
-
-void HAL_RTCEx_WakeUpTimerEventCallback(RTC_HandleTypeDef *hrtc)
-{
+    /* In case of registered handler, call it. */
     if (irq_handler) {
-        // Fire the user callback
         irq_handler();
     }
 }