alsa sound driver for EV3 speaker

I have written an alsa driver for the EV3 speaker. 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.

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.~~ DONE!
3. Add a mute switch to the mixer.