Update MKS SGen-L V2 RGB LED pins (#19762)

Marlin/src/pins/lpc1769/pins_MKS_SGEN_L_V2.h

 // Misc. Functions
 //
 #define LED_PIN                            P1_18  // Used as a status indicator
-#define LED2_PIN                           P1_19
-#define LED3_PIN                           P1_20
-#define LED4_PIN                           P1_21
+
+//
+// RGB LED
+//
+#if ENABLED(RGB_LED)
+  #ifndef RGB_LED_R_PIN
+    #define RGB_LED_R_PIN                  P1_19
+  #endif
+  #ifndef RGB_LED_G_PIN
+    #define RGB_LED_G_PIN                  P1_20
+  #endif
+  #ifndef RGB_LED_B_PIN
+    #define RGB_LED_B_PIN                  P1_21
+  #endif
+#else
+  #define LED2_PIN                         P1_19  // Initialized by HAL/LPC1768/main.cpp
+  #define LED3_PIN                         P1_20
+  #define LED4_PIN                         P1_21
+#endif
 
 /**
  *                _____                                            _____