ASoC: wcd9310: Add the second mixer support to audio RX path

Add the second mixer to audio RX1, RX2, and RX3 path to provide
an option so that the sidetone can be generated from codec
instead of from DSP.

Change-Id: I9ffd5b833abe2d832c4f9b4c9d3ec5ab94fdbde9
Signed-off-by: Kuirong Wang <kuirongw@codeaurora.org>