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-audio-defs.h

@@ -118,4 +118,6 @@
 #define LPASS_BE_RT_PROXY_PCM_RX "PCM_RT_PROXY-RX-1"
 #define LPASS_BE_RT_PROXY_PCM_TX "PCM_RT_PROXY-TX-1"
 
+#define LPASS_BE_PCM_DUMMY_TX_0 "PCM_DUMMY-TX-0"
+
 #endif /*_MSM_AUDIO_DEFS_H*/