ASoC: wcd9xxx: Add support for MAD Slim master channel API

LAB feature is used in listen to capture PCM data once
the keyword is detected. To get the PCM from codec Apps
processor will enable the Slim master channel. PCM data
can be acquired from this master channel which is further
connected to MAD Tx port in codec. This change implements
and exports API to enable the Slim master channel on Apps
side.

Change-Id: Ibac20acc9e1c5c1afe2833c76de894aaf6b1d646
Signed-off-by: Venkat Sudhir <vsudhir@codeaurora.org>
Signed-off-by: Bhalchandra Gajare <gajare@codeaurora.org>