added support for RetroUSB Genesis RetroPort

[Patchy68k]

This is my first ever real code pull request. It's based on Snes9x GX's code for the Mayflash 2-port SNES adapter. I hope that more adapters will be supported in the future.

After looking at my code again, I've noticed these oversights:

1. You can only use up to one RetroUSB adapter at once.
2. I haven't tested this, but I assume when the adapter is plugged in, you won't be able to use a WiiU GamePad.
3. If the RetroUSB adapter is plugged in, neither player can use their analog sticks in-game.
4. You have to use another controller to navigate menus.

That being said, these issues aren't really serious, and there aren't going to be any problems for players who aren't using this adapter. It's you guys' decision whether this is good enough to get into the master branch.

[ekeeke]

Thanks for this pull request, it will indeed be useful to add support for more USB adapters later.
I don't have this USB adapter so I will assume you tested it yourself and it is working fine.
I have added a few remarks on the code changes