[STM32L4] Change LSI_VALUE to 32 kHz (#1662)

* Add DEVICE_RTC_LSI=0

* Change LSI value to 32 kHz

This is the value written in the device datasheet.

* Replace tabulations with spaces

libraries/mbed/targets/hal/TARGET_STM/TARGET_STM32L4/rtc_api.c

@@ -92,8 +92,8 @@ void rtc_init(void)
     if (HAL_RCCEx_PeriphCLKConfig(&PeriphClkInitStruct) != HAL_OK) {
         error("Cannot initialize RTC with LSI\n");
     }
-	// This value is LSI typical value. To be measured precisely using a timer input capture for example.
-	rtc_freq = 40000;
+    // This value is LSI typical value (see device datasheet)
+    rtc_freq = 32000;
 #endif
 
     // Check if RTC is already initialized