hal: add external echo reference to ecns-hfp calls

* Add ec_car_state to switch between single-mic
  and multi-mic ecns hfp calls.
* Add the capability to have an echo reference for
  both single-mic and multi-mic ecns hfp call.
* New snd device is introduced for multi-mic ecns hfp.

Change-Id: I5a8c869cf6854020e15efb61cbaf78f3f027e1ee
Signed-off-by: Guodong Hu <guodhu@codeaurora.org>
Signed-off-by: Derek Chen <chenche@codeaurora.org>