ASoC: Add w->kcontrols, and populate it

Future changes will need reference to the kcontrol created for a given
kcontrol_new. Store the created kcontrol values now.

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