Unmapped on the physical level, not the MMU level.
Fixes booting Game Boy Interface. Previously, Game Boy Interface thought
it was running on a Wii because accessing MEM2 didn't raise a PI
interrupt, and as a result tried to exit to the Homebrew Channel in a
way Dolphin's HLE doesn't recognize. (Dolphin's HLE catches jumps to
0x80001800, but GBI is running without address translation at this point
and therefore jumps to 0x00001800 instead.)