asoc: msm-pcm-routing: add support for a2dp echo ref

Add support for a2dp echo reference for SVA. This is
used to cancel a2dp playback echo from voice activation
input samples.
When phone connects to A2DP speaker like car kit,
selecting A2DP as echo reference can have better VA
performance.

Change-Id: I287ded27421cbda9621f2b2846c3ec8350f080dc
Signed-off-by: Zhou Song <zhous@codeaurora.org>