ASoC: msm8x10-wcd: Add gain controls for ADC gains

Add the mixer controls to control the analog gain on the TX path
of the codec. This gain is applies to the ADC on the path. These
mixer controls will be used by higher layer to set desired gain.

Change-Id: Ic255f83238e8e06d31786c149743ec67cdfd6f87
Signed-off-by: Bhalchandra Gajare <gajare@codeaurora.org>