rk3328_codec: add trivial set_sysclk method implementation

The method does not change any settings, just returns success.

This is to silence "pcm0: failed to set sysclk for codec node" message
printed in audio_soc code.
Previously, that message was printed only during boot which was tolerable.
Recently something changed either in the sound infrastructure or in sound
applications that I use and the message started to be printed regularly.

MFC after: 1 week