ASoC: hdac_hdmi: Add documentation for power management

Add documentation for power management of HDAC HDMI codec device for
various scenarios such as S0/S3, probe and playback use case.

Signed-off-by: Sriram Periyasamy <sriramx.periyasamy@intel.com>
Signed-off-by: Sanyog Kale <sanyog.r.kale@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>