CRAS software dsp: Add a mix channel builtin module

Stumpy has a mono speaker but report itself as stereo. Add
a module with the functionality to mix two channels into one.

BUG=chromium-os:37880
TEST=Try speaker test left/right youtube, can hear both channel now.

Change-Id: I5fced006a7348e0c6f6ceecf496a05806ec4c0f7
Reviewed-on: https://gerrit.chromium.org/gerrit/41735
Reviewed-by: Chih-Chung Chang <[email protected]>
Commit-Queue: Hsinyu Chao <[email protected]>
Reviewed-by: Hsinyu Chao <[email protected]>
Tested-by: Hsinyu Chao <[email protected]>
5 files changed