ASoC: dapm: Implement stereo mixer control support

While DAPM is mono or single channel, its controls can be shared between
widgets, such as sharing one stereo mixer control between the left and
right channel widgets. An example such as the following routes

    [Line In Left]----------<Line In Playback Switch>-------[Left Mixer]
                                          ^
          ^           ^                   |                      ^
       (inputs)    (paths)   <shared stereo mixer control>   (outputs)
          v           v                   |                      v
                                          v
    [Line In Right]---------<Line In Playback Switch>-------[Right Mixer]

where we have separate widgets and paths for the left and right channels
from "Line In" to "Mixer", but a shared stereo mixer control for the
2 paths.

This patch introduces support for such shared mixer controls, allowing
more than 1 path to be attached to a single stereo control, and being
able to control left/right channels independently.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Mark Brown <broonie@kernel.org>