ASoC: Move DAI structure definitions into new soc-dai.h

ASoC v2 factors most of the contents of soc.h out into separate headers,
including soc-dai.h for the DAI. Factor the existing DAI API out into
this file in order to prepare for backporting of the ASoC v2 DAI API.
Also backport some of Liam's improvements to the documentation.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>