Implement write-only bits in sound registers FF10-FF2F, including non-registers in that range.

References:
* Blargg's CGB audio tests, 01-registers #2 "NR10-NR51 and wave RAM write/read"..
* Blargg's CGB audio tests, 01-registers #3 "NR52 write/read".
Clear registers in I/O RAM when disabling the APU.

References:
* Blargg's CGB audio tests, 01-registers #5 "Powering APU off should write 0 to all regs".
Ignore writes to registers FF10-FF25 while the APU is off.

References:
* Blargg's CGB audio tests, 01-registers #6 "When off, should ignore writes to registers".
Merge pull request #6 from Nebuleon/correctness

Emulation correctness: Part 1, audio registers