Add support for LC-3 MMIO devices

[sameer-s]

• [ ] KBSR
• [ ] KBDR
• [ ] DSR
• [ ] DDR
• [ ] PSR
• [ ] MCR

[sameer-s]

All GBA devices have been added. Adding LC-3 devices is low priority, since they aren't needed for core functionality.

[sameer-s]

Not needed at the moment as the scope of the simulator is limited to user-mode.