fixup! ASoC: SOF: ipc4: implement pm ctx_save callback

Add comment for ctx_save and note the non existent ctx_restore callback
implementation

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@linux.intel.com>

Author: ujfalusi

sound/soc/sof/ipc4.c

@@ -618,6 +618,15 @@ static int sof_ipc4_set_core_state(struct snd_sof_dev *sdev, int core_idx, bool
 	return sof_ipc4_tx_msg(sdev, &msg, msg.data_size, NULL, 0, false);
 }
 
+/*
+ * The context save callback is used to send a message to the firmware notifying
+ * it that the primary core is going to be turned off, which is used as an
+ * indication to prepare for a full power down, thus preparing for IMR boot
+ * (when supported)
+ *
+ * Note: in IPC4 there is no message used to restore context, thus no context
+ * restore callback is implemented
+ */
 static int sof_ipc4_ctx_save(struct snd_sof_dev *sdev)
 {
 	return sof_ipc4_set_core_state(sdev, SOF_DSP_PRIMARY_CORE, false);