ASoC: mxs: mxs-pcm does not need to be a plaform_driver

Same as the commit 518de86b (ASoC: tegra: register 'platform' from DAIs,
get rid of pdev), it makes mxs-pcm not a platform_driver but helper to
register "platform", so that the platform_device for mxs-pcm can be
saved completely.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>