ASoC: SOF: make ctx_store and ctx_restore as optional

Commit 657774acd0 ("ASoC: SOF: Make sof_suspend/resume IPC agnostic")
did not marked ctx_store and ctx_restore as Optional.

Fixes: 657774acd0 ("ASoC: SOF: Make sof_suspend/resume IPC agnostic")
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@linux.intel.com>
Reviewed-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Reviewed-by: Bard Liao <yung-chuan.liao@linux.intel.com>
Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20220610083549.16773-2-peter.ujfalusi@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>

sound/soc/sof/sof-priv.h

@@ -376,8 +376,8 @@ struct sof_ipc_fw_tracing_ops {
 
 /**
  * struct sof_ipc_pm_ops - IPC-specific PM ops
- * @ctx_save:		Function pointer for context save
- * @ctx_restore:	Function pointer for context restore
+ * @ctx_save:		Optional function pointer for context save
+ * @ctx_restore:	Optional function pointer for context restore
  */
 struct sof_ipc_pm_ops {
 	int (*ctx_save)(struct snd_sof_dev *sdev);