ASoC: msm: Add external echo reference support for audio path

If the audio mixing is done within codec, then echo reference
needs to be obtained from the mixed output in the codec.
Add support to get the external echo reference from SLIMBUS_1_TX
afe port.

Change-Id: I046172c02dc2eac5d1b8156abca3e4ca960d78f4
Signed-off-by: Phani Kumar Uppalapati <phaniu@codeaurora.org>