powerpc/fsl: Use new clockgen binding

The driver retains compatibility with old device trees, but we don't
want the old nodes lying around to be copied, or used as a reference
(some of the mux options are incorrect), or even just being clutter.

Signed-off-by: Scott Wood <oss@buserror.net>
Signed-off-by: Tang Yuantian <andy.tang@nxp.com>
[scottwood: removed sysclk node added by Andy]
Signed-off-by: Scott Wood <oss@buserror.net>