Allow pins overrides for SKR V1.3 (#15451)

Marlin/src/pins/lpc1768/pins_BIGTREE_SKR_V1.3.h

 //
 // Servos
 //
-#define SERVO0_PIN         P2_00
+#ifndef SERVO0_PIN
+  #define SERVO0_PIN       P2_00
+#endif
 
 //
 // Limit Switches
 //
 // Heaters / Fans
 //
-#define HEATER_0_PIN       P2_07
+#ifndef HEATER_0_PIN
+  #define HEATER_0_PIN     P2_07
+#endif
 #if HOTENDS == 1
-  #define FAN1_PIN         P2_04
+  #ifndef FAN1_PIN
+    #define FAN1_PIN       P2_04
+  #endif
 #else
-  #define HEATER_1_PIN     P2_04
+  #ifndef HEATER_1_PIN
+    #define HEATER_1_PIN   P2_04
+  #endif
+#endif
+#ifndef FAN_PIN
+  #define FAN_PIN          P2_03
+#endif
+#ifndef HEATER_BED_PIN
+  #define HEATER_BED_PIN   P2_05
 #endif
-#define FAN_PIN            P2_03
-#define HEATER_BED_PIN     P2_05
 
 /**
  *              _____                                             _____