alsa sound driver for EV3 speaker

For my next trick... I will pull a rabbit out of a hat. No wait, I will write an alsa driver for the EV3 speaker.

Well, here it is. The sound quality is pretty terrible, but I think that is more of a hardware design issue that anything. If anyone has any brilliant ideas on how we could improve the quality, I am all ears. (Pun sort of intended - its getting late and I'm getting silly.)

On the TODO list:
1. There are some pops when playback starts and when you adjust the volume control too fast. Hopefully we can figure out how to get rid of these.
2. If we want a tone mode like the official LEGO firmware, we could probably do something like the digital beep device in the alsa hda driver.
3. Add a mute switch to the mixer.