Add buttonmap.xml and topology.xml

[garbear]

From FCEUmm source:

https://github.com/libretro/libretro-fceumm/blob/master/src/drivers/libretro/libretro.c

TODO:

• [x] Add support for "maxplayers" attribute in game.libretro.

TODO: Add the following NES controllers:

• [ ] NES Zapper
• [ ] Arkanoid Controller
• [ ] Famicom Arkanoid Controller
• [ ] Oeka Kids Tablet
• [ ] Bandai Hyper Shot