ASoC: fsl_ssi: Refine all comments

This patch refines the comments by:
1) Removing all out-of-date comments
2) Removing all not-so-useful comments
3) Unifying the styles of all comments
4) Shortening comments to be more conise
5) Adding comments to improve code readablity
6) Moving all register related comments to fsl_ssi.h
7) Adding comments to all register and field defines

Signed-off-by: Nicolin Chen <nicoleotsuka@gmail.com>
Tested-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
Reviewed-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
Acked-by: Timur Tabi <timur@tabi.org>
Signed-off-by: Mark Brown <broonie@kernel.org>