ALSA: doc: ReSTize alsa-driver-api document

A simple conversion of alsa-driver-api document from DocBook to ReST.

It's moved to the new Documentation/sound/kernel-api subdirectory that
will contain other ALSA kernel API documents.

The GPL legal note was removed, as it's superfluous (and doesn't fit
with ReST kernel docs pretty well).

Signed-off-by: Takashi Iwai <tiwai@suse.de>