ASoC: msm: qdsp6v2: Add SLIM_1_TX as external echo reference port

Add support for SLIM_1_TX as external echo reference port for voice
session. This port is used as far end AFE port in echo cancellation
when Vbat monitor is enabled.

Change-Id: I0f6b17d227b210a0694440e481ba21794b84b58b
Signed-off-by: Venkata Narendra Kumar Gutta <vgutta@codeaurora.org>