Add GPIO to toggle /TIME signal on megadrive shield

[tehKaiN]

/TIME is NOT clock. It's pin B31, and is low when the CPU accesses 0xA13000 through 0xA130FF.

It's used to toggle SRAM and other stuff in megadrive carts. This way it'll be possible to dump multicarts and SRAM saves.

• [ ] Add to rev1 using wire
• [ ] Implement features needed in firmware & software
• [ ] Add to schematic & PCB

Some info about banking:

http://forums.nesdev.com/viewtopic.php?t=11615 http://devster.monkeeh.com/sega/radica/

[tehKaiN]

this could be done using 74 chips on address lines, but the less components the better. And there's still plenty of GPIOs for use.