Add fix to reset low power timer when set_time() is called otherwise you just end up adding to the time instead of setting it.

ARMmbed#7849

Author: RobMeades

platform/mbed_rtc_time.cpp

@@ -55,6 +55,7 @@ static time_t _rtc_lpticker_read(void)
 
 static void _rtc_lpticker_write(time_t t)
 {
+    _rtc_lp_timer->reset();
     _rtc_lp_base = t;
 }