dsp: add support to set custom channel mixer coefficients

Enhance Channel Mixer control for MultiMedia Frontends to
get custom channel mixer coefficients from userspace and
use it during device routings to control stream to device
channel mappings.

Change-Id: Ifedac25de5ae6bad1092b66ec405bba66a80f608
Signed-off-by: Cong Tang <congt@codeaurora.org>
Signed-off-by: Dhananjay Kumar <dhakumar@codeaurora.org>