Add Einstart-S support (#11219)

Indeed the HAL does not need to be mucked around with to expose ATmega2560 pins not available as numbers on the MEGA board, I'll need to update the wiki with that tidbit and a reference to the pin-mapping comment in `fastio_1280.h`.

Marlin/src/core/boards.h

@@ -104,6 +104,7 @@
 #define BOARD_SCOOVO_X9H        321   // abee Scoovo X9H
 #define BOARD_GT2560_REV_A      74    // Geeetech GT2560 Rev. A
 #define BOARD_GT2560_REV_A_PLUS 75    // Geeetech GT2560 Rev. A+ (with auto level probe)
+#define BOARD_EINSTART_S        666   // Einstart retrofit
 
 //
 // ATmega1281, ATmega2561