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.

Change-Id: I461b043e9a5e888667ca97eac403a7286eef4f60
Signed-off-by: Venkat Sudhir <vsudhir@codeaurora.org>