Make generic ASoC AC'97 CODEC selectable in config.

This way this driver can be used for platforms which don't need
specialized AC'97 CODEC drivers but which are not directly
selectable in config themselves (for example DT based ones).
Signed-off-by: Maciej Szmigiero <mail@maciej.szmigiero.name>
Signed-off-by: Mark Brown <broonie@kernel.org>