[NUCLEO_F303K8] Enable LowPowerTimer

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

Change-Id: I4543b0f9d6995b1c90b766f4909e3a324db98022

Author: BartSX

hal/targets/hal/TARGET_STM/TARGET_STM32F3/TARGET_NUCLEO_F303K8/device.h

@@ -48,12 +48,14 @@
 #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