asoc: Add dummy PCM backend

Add dummy PCM backend definition for virtual devices.
This is being added to support echo reference capture.

Change-Id: I7830f9adf963397d06b837add3f9e1c0fe31563c

asoc/msm_dailink.h

@@ -1,6 +1,7 @@
 /* SPDX-License-Identifier: GPL-2.0-only */
 /*
  * Copyright (c) 2020-2021, The Linux Foundation. All rights reserved.
+ * Copyright (c) 2022 Qualcomm Innovation Center, Inc. All rights reserved.
  */
 
 #include <sound/soc.h>
@@ -15,6 +16,11 @@ SND_SOC_DAILINK_DEFS(usb_audio_tx,
 	DAILINK_COMP_ARRAY(COMP_DUMMY()),
 	DAILINK_COMP_ARRAY(COMP_PLATFORM("snd-soc-dummy")));
 
+SND_SOC_DAILINK_DEFS(pcm_dummy_tx0,
+	DAILINK_COMP_ARRAY(COMP_CPU("snd-soc-dummy-dai")),
+	DAILINK_COMP_ARRAY(COMP_DUMMY()),
+	DAILINK_COMP_ARRAY(COMP_PLATFORM("snd-soc-dummy")));
+
 SND_SOC_DAILINK_DEFS(slimbus_7_rx,
 	DAILINK_COMP_ARRAY(COMP_CPU("snd-soc-dummy-dai")),
 	DAILINK_COMP_ARRAY(COMP_CODEC("btfmslim_slave",