ASoC: hdac_hdmi: support 'ELD' mixer

Add an binary mixer 'ELD' to each HDMI PCM device so user space
could read the ELD data of external HDMI display.

Signed-off-by: Brent Lu <brent.lu@intel.com>
Link: https://lore.kernel.org/r/20200818004413.12852-1-brent.lu@intel.com


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