Add the sync_audio_rate callback.

With the callback, audio driver can trigger
i915 driver to set the proper N/CTS or N/M
based on different sample rates.
Signed-off-by: Libin Yang <libin.yang@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>