powerpc/fsl: Add #clock-cells and clockgen label to clockgen nodes

This allows new-style clock references to be used, which is needed for
fman.  The old clock nodes will be removed and all clock references
converted to new-style once the qoriq-cpufreq driver is updated to stop
depending on the old-style references in cpu nodes.

Signed-off-by: Scott Wood <scottwood@freescale.com>