ASoC: msm: Add support for customized stereo mixing in DSP

For the use case of playback through tunnel mode, customized
mixing for stereo content is required for hearing impaired
Add support to enable customized stereo mixing in DSP.
The customized mixing supported is the average of left and
right channel and further duplicating the averaged output to
both left and right channels.

Change-Id: I8639d1e298a24a76a32fffdc2c0e372a48aeeed0
Signed-off-by: Jitendra Singh Naruka <jnaruk@codeaurora.org>