ASoC: Add WM8958 VSS support

With appropriate firmware the WM8958 can support Virtual Surround Sound or
VSS, widening the stereo audio image for improved user experience. Enable
support for this mode of operation when the appropriate firmware can be
loaded at runtime.

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