ASoC: msm: qdsp6v2: add support for ADM_OPEN_V6 for multi-mic ec

Add support for ADM_CMD_DEVICE_OPEN_V6 when the mixer path
specifies to use the multi-mic-echo-reference, which configures
the EndPoint2 for Echo Reference.

CRs-fixed: 1022080
Change-Id: I474f39a3437fa18003f4342e003d689b95837699
Signed-off-by: Derek Chen <chenche@codeaurora.org>
Signed-off-by: Banajit Goswami <bgoswami@codeaurora.org>