added shift register pins for reprapworld keypad v1.1

I recently upgraded my Marlin to V1.1.x and decided to publish my fix to make marlin compile properly when #define board megatronics 2 in combination with #define reprapworld keypad v1.1. It was previously described in the comments of a closed pull request. MarlinFirmware#472. All the buttons on the keypad are now operational.