[DISCO_L053C8] Enable LowPowerTimer

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

Change-Id: I2e3ff6c7dfbe709cc39bf6ebe86266031532f90b

Author: BartSX

hal/targets/hal/TARGET_STM/TARGET_STM32L0/TARGET_DISCO_L053C8/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