ASoC: msm8226: Support multichannel playback over proxy port

Add kcontrol to allow configuring the proxy backend to six channels.
This is to support multichannel playback over proxy port on 8x26.

Change-Id: Ib2994edadbb2e1afcace1f058c21458c4b82d6ea
Signed-off-by: Krishnankutty Kolathappilly <kkolat@codeaurora.org>