ASoC: SOF: add runtime idle callback

Add ability to implement a SOF device level runtime idle callback.

Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Reviewed-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/20190702132428.13129-3-kai.vehmanen@linux.intel.com


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