Teensy 4.1 Ethernet support (#19801)

Marlin/Configuration_adv.h

@@ -3456,6 +3456,13 @@
                               // Default behavior is limited to Z axis only.
 #endif
 
+/**
+ * Ethernet. Use M552 to enable and set the IP address.
+ */
+#if HAS_ETHERNET
+  #define MAC_ADDRESS { 0xDE, 0xAD, 0xBE, 0xEF, 0xF0, 0x0D }  // A MAC address unique to your network
+#endif
+
 /**
  * WiFi Support (Espressif ESP32 WiFi)
  */