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>

sound/soc/sof/sof-acpi-dev.c

@@ -116,7 +116,7 @@ static const struct sof_dev_desc sof_acpi_cherrytrail_desc = {
 static const struct dev_pm_ops sof_acpi_pm = {
 	SET_SYSTEM_SLEEP_PM_OPS(snd_sof_suspend, snd_sof_resume)
 	SET_RUNTIME_PM_OPS(snd_sof_runtime_suspend, snd_sof_runtime_resume,
-			   NULL)
+			   snd_sof_runtime_idle)
 };
 
 static void sof_acpi_probe_complete(struct device *dev)