ASoC: wcd934x: add handset speaker gain mixer control

In msmcobalt platform, speaker path is used for handset mode also.
Add the mixer control to change gain in handset mode.

Change-Id: Ia291f7449cd04a855102a03324f7cdcb6f13dad0
Signed-off-by: Vidyakumar Athota <vathota@codeaurora.org>