asoc: add support for custom channel map

In a stream we need to add support for downmixed stereo channel
along with the left and right channels. Add custom channel map
to support downmixed stereo channels.

Change-Id: Icb7fa226a6f07ee4f7d00b8f55fe578c27719c86
Signed-off-by: Surendar Karka <skarka@codeaurora.org>