E2 pins for GT 2560 v3 (#16016)

Marlin/src/pins/mega/pins_GT2560_V3.h

 #define E1_DIR_PIN         47
 #define E1_ENABLE_PIN      48
 
+#define E2_STEP_PIN        43
+#define E2_DIR_PIN         45
+#define E2_ENABLE_PIN      41
+
 //
 // Temperature Sensors
 //