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

Same as the commit 518de86 (ASoC: tegra: register 'platform' from DAIs,
get rid of pdev). It makes davinci-pcm not a platform_driver but helper
to register "platform", so that the platform_device for davinci-pcm can
be saved completely.

Signed-off-by: Hebbar, Gururaja <gururaja.hebbar@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>