Add external closed loop controller support

Marlin/src/gcode/gcode.h

@@ -76,6 +76,7 @@
  * M3   - Turn laser/spindle on, set spindle/laser speed/power, set rotation to clockwise
  * M4   - Turn laser/spindle on, set spindle/laser speed/power, set rotation to counter-clockwise
  * M5   - Turn laser/spindle off
+ * M12  - Set up closed loop control system. More features coming soon. (Requires EXTERNAL_CLOSED_LOOP_CONTROLLER)
  * M17  - Enable/Power all stepper motors
  * M18  - Disable all stepper motors; same as M84
  * M20  - List SD card. (Requires SDSUPPORT)
@@ -438,6 +439,10 @@ private:
     static void M5();
   #endif
 
+  #if ENABLED(EXTERNAL_CLOSED_LOOP_CONTROLLER)
+    static void M12();
+  #endif
+
   static void M17();
 
   static void M18_M84();