asoc: 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.
Also increase the ec ref channel count support to Sixteen from
current max of Eight.

Change-Id: I2297508aedd177580b1c9b2bed3c7c8421507629
Signed-off-by: Dhananjay Kumar <dhakumar@codeaurora.org>