stm32/system_stm32: Fix LSE initialization for STM32H7/F4/F7.

The LSE oscillator was never being enabled for STM32H7, F4, and F7
families. The code would set the RTC clock source to LSE via
HAL_RCCEx_PeriphCLKConfig() but never called HAL_RCC_OscInit() with
RCC_OSCILLATORTYPE_LSE to actually enable the oscillator.

This fix adds LSE oscillator initialization following the pattern
already used successfully on STM32L4 boards. When MICROPY_HW_RTC_USE_LSE
is enabled, the LSE is now properly initialized before being selected as
the RTC clock source.

Verified on STM32H747I-DISCO: LSEON=1, LSERDY=1, RTCSEL=LSE.

Signed-off-by: Andrew Leech <[email protected]>

Author: pi-anl

ports/stm32/system_stm32.c

@@ -249,6 +249,11 @@ MP_WEAK void SystemClock_Config(void) {
     RCC_OscInitStruct.OscillatorType |= RCC_OSCILLATORTYPE_HSI48;
     #endif
 
+    #if MICROPY_HW_RTC_USE_LSE
+    RCC_OscInitStruct.LSEState = RCC_LSE_ON;
+    RCC_OscInitStruct.OscillatorType |= RCC_OSCILLATORTYPE_LSE;
+    #endif
+
     RCC_OscInitStruct.PLL.PLLSource = MICROPY_HW_RCC_PLL_SRC;
 
     #elif defined(STM32L4)