ASoC: Let snd_soc_platform subclass snd_soc_component

There is an increasing amount of code that is very similar between platforms,
CODECS and other components. Making platforms a component will allow us to
share this code. For now the patch just adds component and component_driver
fields to the platform and platform_driver structs and registers the platform as
a component. Followup patches will be used to consolidate code between the
different types of components.

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