ASoC: Allow Freescale MPC8610 audio drivers to be compiled as modules

Change the Kconfig and Makefile options for Freescale MPC8610 audio drivers
so that they can be compiled as modules, and simplify the Kconfig choices
so that only the platform is selected.

Also fix the naming of the driver files to conform to ALSA standards.

[Removed extraneous SND_SOC dependency -- broonie]

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>