ASoC: msm: Add support for external EC reference point

Add mux to sound card topology with output of mux routed
to CS voice uplink front-end as way to indicate external
EC reference point. Currently, MI2S_TX is the only
AFE input port defined for this MUX. This change is required
if external EC is to be used with external speaker.

Change-Id: Iee7a996f782dc15fd1b8cc14e4914c9e52489d90
Signed-off-by: Venkat Sudhir <vsudhir@codeaurora.org>