stm32h7 dual core: allow override of HAL_RCC_GetSysClockFreq

It's the only needed entry point to get the right timings for all peripherals

Author: facchinm

Diff for: targets/TARGET_STM/TARGET_STM32H7/STM32Cube_FW/STM32H7xx_HAL_Driver/stm32h7xx_hal_rcc.c

@@ -1354,7 +1354,7 @@ void HAL_RCC_DisableCSS(void)
   *
   * @retval SYSCLK frequency
   */
-uint32_t HAL_RCC_GetSysClockFreq(void)
+__attribute__((weak)) uint32_t HAL_RCC_GetSysClockFreq(void)
 {
   uint32_t pllp, pllsource, pllm, pllfracen, hsivalue;
   float_t fracn1, pllvco;