[NES II] Select RAM at $c000-$dfff range

[JMarlin]

Requires prior completion of #37 to provide room for extra 8K of RAM in PC mode

• [ ] RAM at $8000-$b000 operates unchanged
• [ ] Cart addresses $c000-$dfff map to RAM locations $8000-$9fff in all modes
• [ ] RAM enable is triggered in $c000-$dfff range