Add pins for Dual Z UART on SKR 1.3 (#13474)

Fix #13470

Marlin/src/pins/pins_BIGTREE_SKR_V1.3.h

@@ -140,6 +140,10 @@
 
   #define E1_SERIAL_TX_PIN P1_04
   #define E1_SERIAL_RX_PIN P1_01
+
+  #define Z2_SERIAL_TX_PIN P1_04
+  #define Z2_SERIAL_RX_PIN P1_01
+
 #endif
 
 //