ASoC: Document DAI signal polarity

Currently there is no clear definition of what FSYNC polarity is.
Different drivers use its own definition of what is "normal" and what is
"inverted" fsync. This leads to compatibility problems between drivers.

For example TegraX1 driver assumes that DSP-A format with frames
starting at rising FSYNC edge has "inverted" polarity,
while RT5677 assumes it is "normal" polarity.

Explicitly specify meaning of BCLK/FSYNC polarity to avoid future
compatibility problems.

Signed-off-by: Anatol Pomozov <anatol.pomozov@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>