ASoC: Add snd_soc_kcontrol_platform() helper function

For platform controls snd_kcontrol_chip() currently returns a pointer to the
platform that registered the control. With the upcoming consolidation of
platform and CODEC controls this will change. Prepare for this by introducing
the snd_soc_kcontrol_platform() helper function that will hide the
implementation details of how the platform for a control can be obtained. This
will allow us to change this easily in the future.

The patch also updates all platforms to use this new helper function.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@linaro.org>