MMU2 as standard serial device (#20771)

Marlin/src/HAL/STM32/HAL.h

@@ -65,6 +65,16 @@
   #endif
 #endif
 
+#ifdef MMU2_SERIAL_PORT
+  #if MMU2_SERIAL_PORT == -1
+    #define MMU2_SERIAL SerialUSB
+  #elif WITHIN(MMU2_SERIAL_PORT, 1, 6)
+    #define MMU2_SERIAL MSERIAL(MMU2_SERIAL_PORT)
+  #else
+    #error "MMU2_SERIAL_PORT must be -1 or from 1 to 6. Please update your configuration."
+  #endif
+#endif
+
 #ifdef LCD_SERIAL_PORT
   #if LCD_SERIAL_PORT == -1
     #define LCD_SERIAL SerialUSB

Marlin/src/HAL/STM32/MarlinSerial.cpp

@@ -48,6 +48,10 @@
   DECLARE_SERIAL_PORT_EXP(SERIAL_PORT_2)
 #endif
 
+#if defined(MMU2_SERIAL_PORT) && MMU2_SERIAL_PORT >= 0
+  DECLARE_SERIAL_PORT_EXP(MMU2_SERIAL_PORT)
+#endif
+
 #if defined(LCD_SERIAL_PORT) && LCD_SERIAL_PORT >= 0
   DECLARE_SERIAL_PORT_EXP(LCD_SERIAL_PORT)
 #endif