Change default Re-ARM UART pin order (#15037)

Marlin/src/pins/lpc1768/pins_RAMPS_RE_ARM.h

   // P2_08 E1-Step
   // P2_13 E1-Dir
 
-  #define X_SERIAL_TX_PIN  P2_13
-  #define X_SERIAL_RX_PIN  P2_13
+  #ifndef  X_SERIAL_TX_PIN
+    #define X_SERIAL_TX_PIN  P0_01
+  #endif
+  #ifndef X_SERIAL_RX_PIN
+    #define X_SERIAL_RX_PIN  P0_01
+  #endif
 
-  #define Y_SERIAL_TX_PIN  P0_00
-  #define Y_SERIAL_RX_PIN  P0_00
+  #ifndef Y_SERIAL_TX_PIN
+    #define Y_SERIAL_TX_PIN  P0_00
+  #endif
+  #ifndef Y_SERIAL_RX_PIN
+    #define Y_SERIAL_RX_PIN  P0_00
+  #endif
 
-  #define Z_SERIAL_TX_PIN  P0_01
-  #define Z_SERIAL_RX_PIN  P0_01
+  #ifndef Z_SERIAL_TX_PIN
+    #define Z_SERIAL_TX_PIN  P2_13
+  #endif
+  #ifndef Z_SERIAL_RX_PIN
+    #define Z_SERIAL_RX_PIN  P2_13
+  #endif
 
-  #define E0_SERIAL_TX_PIN P2_08
-  #define E0_SERIAL_RX_PIN P2_08
+  #ifndef E0_SERIAL_TX_PIN
+    #define E0_SERIAL_TX_PIN P2_08
+  #endif
+  #ifndef E0_SESIAL_RX_PIN
+    #define E0_SERIAL_RX_PIN P2_08
+  #endif
 
 #endif