[NUCLEO_F746ZG] Enable LowPowerTimer

We need to enable the LSI clock to be able to start the RTC periodic unit.

Change-Id: Ic7518dfcf5153f925c7c030f8de0259e19aab963

Author: BartSX

hal/targets/hal/TARGET_STM/TARGET_STM32F7/TARGET_NUCLEO_F746ZG/device.h

@@ -49,12 +49,14 @@
 #define DEVICE_SPISLAVE         1
 
 #define DEVICE_RTC              1
-#define DEVICE_RTC_LSI          0
+#define DEVICE_RTC_LSI          1
 
 #define DEVICE_PWMOUT           1
 
 #define DEVICE_SLEEP            1
 
+#define DEVICE_LOWPOWERTIMER    1
+
 //=======================================
 
 #define DEVICE_SEMIHOST         0