powerpc/boot/dts: drop obsolete 'fsl5200-clocking'

The 'fsl5200-clocking'-property was dropped since
0d1cde23

. Remove all occurences
in dts-files.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>