Make opcode cycles into a proper initialised array.

Shrinks gameyob.nds by 2 KiB.
mmu.cpp: readMemory, writeMemory: Constrain the range of the Game Boy memory address so that 'gcc' issues a jump table for the switch instead of a binary search. Adds 1 left shift, but removes 3 comparisons and 3 conditional jumps.
Move various hotspots that are too large for the ARM9 cache into the ARM9 ITCM (opcodes, memory accessors) and DTCM (opcode cycle count arrays).
Merge pull request #1 from Nebuleon/optimisations

Optimisations
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
Add more hotspots to the ITCM. Add the I/O registers to the DTCM.
Make the "no game loaded" upper screen black because why not.
Revert "Make the "no game loaded" upper screen black because why not." It was for the wrong thing: it was for the times during a loaded game that the Game Boy screen is not drawn due to VRAM changes.

This reverts commit ec54f757113f8e88fbabd331dd526bab3633f9a6.
gbgfx.cpp: Avoid copying the drawing state onto the stack in drawLine. Has an impact on wavy screens, not static.
Designate sound registers by byte offset to FF00 instead of by 16-bit address.
gbcpu.cpp: Initialise I/O registers with writeIO in initCPU.
Video register code moved to gbgfx.cpp
Removed some residual gbgfx functions
Rearranged graphics register handlers a bit
Optimise writes of the PC register to the stack by using a union of bytes for it instead of >>8 and &0xFF. May help in some interrupt-heavy cases.
gbcpu.cpp: Flags are often used, too. Put them in a local variable.
gbcpu.cpp: Optimise away 'LD Register, Self'; reduce the instruction count for 'XOR A, A', 'SUB A, A', 'AND A, A' and 'OR A, A'.
gbcpu.cpp: Optimise everything that has to do with the accumulator register, A.