Add MKS MINI 12864 support for GT2560

Marlin/example_configurations/Geeetech/GT2560/Configuration.h

 /**
  * Customize common displays for GT2560
  */
-#if ENABLED(ULTIMAKERCONTROLLER) || ENABLED(REPRAP_DISCOUNT_SMART_CONTROLLER) || ENABLED(G3D_PANEL)
+#if ENABLED(ULTIMAKERCONTROLLER) || ENABLED(REPRAP_DISCOUNT_SMART_CONTROLLER) || ENABLED(G3D_PANEL) || ENABLED(MKS_MINI_12864)
   #define SDSUPPORT   // Force SD Card support on for these displays
 #elif ENABLED(ULTRA_LCD) && ENABLED(DOGLCD) // No panel, just graphical LCD?
   #define LCD_WIDTH 20 // Default is 22. For this Geeetech use 20

Marlin/pins_GT2560_REV_A.h

 
   #if ENABLED(NEWPANEL)
 
-    #define LCD_PINS_RS    20
-    #define LCD_PINS_ENABLE 17
-    #define LCD_PINS_D4    16
-    #define LCD_PINS_D5    21
-    #define LCD_PINS_D6     5
-    #define LCD_PINS_D7     6
+  #if ENABLED(MKS_MINI_12864)
+      #define DOGLCD_A0         5
+      #define DOGLCD_CS         21
+
+      #define BTN_EN1           40
+      #define BTN_EN2           42
+      #define BTN_ENC           19
+
+      #define SDSS              53
+      #define SD_DETECT_PIN     38
+  #else
+
+      #define LCD_PINS_RS    20
+      #define LCD_PINS_ENABLE 17
+      #define LCD_PINS_D4    16
+      #define LCD_PINS_D5    21
+      #define LCD_PINS_D6     5
+      #define LCD_PINS_D7     6
+
+      // Buttons are directly attached
+      #define BTN_EN1        42
+      #define BTN_EN2        40
+      #define BTN_ENC        19
 
-    // Buttons are directly attached
-    #define BTN_EN1        42
-    #define BTN_EN2        40
-    #define BTN_ENC        19
+      #define SD_DETECT_PIN  38
 
-    #define SD_DETECT_PIN  38
+  #endif
 
   #else // !NEWPANEL