ASoC: intel, keembay-i2s: Add info for device to use DMA

Add descriptions for entries needed for audio device to use DMA
channels for audio playback and capture.

Signed-off-by: Michael Sit Wei Hong <michael.wei.hong.sit@intel.com>
Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20210108031248.20520-5-michael.wei.hong.sit@intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>