ASoC: Add 3D stereo support for wm8996

My first patch to ASoC ever! If I did something wrong, blame Ian.

Signed-off-by: Susan Gao <sgao@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>