dsp: add support to configure MFC in ec ref path

Add support to process ec at channel count other than the
playback channel count by providing ability to set downmixed
channel count and downmixing weights to be used to downmix
ec ref data obtained from playback path.
These configs would be used to explicitly configure MFC module
present in ec ref path, for usecases where ec processing cannot
be done at playback channel count due to algorithmic or resource
limitations.

Change-Id: Iaa80a0f0da3697c55751b2201578252d6466d3b5
Signed-off-by: Dhananjay Kumar <dhakumar@codeaurora.org>