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/pins/pins.h

@@ -183,6 +183,8 @@
   #include "pins_GT2560_REV_A.h"      // ATmega1280, ATmega2560                     env:megaatmega1280 env:megaatmega2560
 #elif MB(GT2560_REV_A_PLUS)
   #include "pins_GT2560_REV_A_PLUS.h" // ATmega1280, ATmega2560                     env:megaatmega1280 env:megaatmega2560
+#elif MB(EINSTART_S)
+  #include "pins_EINSTART-S.h"        // ATmega1280, ATmega2560                     env:megaatmega1280 env:megaatmega2560
 
 //
 // ATmega1281, ATmega2561