configs: add new mixer paths for mmsecns hfp calls

* <hfp-sco-wb bt-sco-mmsecns> and <hfp-sco bt-sco-mmsecns>
  are used for multi-mic ecns hfp calls.
* <hfp-sco-wb bt-sco-wb> and <hfp-sco bt-sco> are used for
  single-mic ecns hfp calls;
* Provide the capability to overide backend_tag for paths.

Change-Id: Ibae794b59e5e4fae8673ce1a95845c5f11f25b77
Signed-off-by: Guodong Hu <guodhu@codeaurora.org>