K64F: adapt RTC drivers to the new standards (free function)

rtc_free() does not stop the RTC from counting. Only disables clock gate since processor no longer needs to read RTC registers.

targets/TARGET_Freescale/TARGET_MCUXpresso_MCUS/api/rtc_api.c

@@ -37,7 +37,10 @@ void rtc_init(void)
 
 void rtc_free(void)
 {
-    RTC_Deinit(RTC);
+#if !(defined(FSL_SDK_DISABLE_DRIVER_CLOCK_CONTROL) && FSL_SDK_DISABLE_DRIVER_CLOCK_CONTROL)
+    /* Gate the module clock */
+    CLOCK_DisableClock(kCLOCK_Rtc0);
+#endif /* FSL_SDK_DISABLE_DRIVER_CLOCK_CONTROL */
 }
 
 /*