ASoC: Add documentation for KeemBay i2s

Document Intel KeemBay i2s DT bindings.

Signed-off-by: Sia Jee Heng <jee.heng.sia@intel.com>
Link: https://lore.kernel.org/r/1591682783-1923-4-git-send-email-jee.heng.sia@intel.com


Signed-off-by: Mark Brown <broonie@kernel.org>