ASoC: msm: Add support for external EC reference from APQ.

Add support for a back-end DAI that routes the mixed audio and
voice streams from the application processor to the external
modem for echo-cancellation.

Change-Id: I6c938803dccdb6ed8e15e870cceb6b0de7019afb
Signed-off-by: Neema Shetty <nshetty@codeaurora.org>
7 files changed