Put the registers in guaranteed contiguous memory. Also move them to the DTCM.

The binary has grown by 512 bytes.
mmu.cpp: Split readMemory and writeMemory into readIO and writeIO for the Game Boy I/O registers at FFxx hex. Use them in the opcodes that deal exclusively with I/O registers at (FF00 + immediate).
Merge pull request #2 from Nebuleon/optimisations

Optimisations