[POWERPC] fsl_soc.c cleanup

1. Update the way get_brgfreq() finds things in the device tree.

It now uses names that are less namespace polluting.  The old names
are supported until all boards are converted.

2. "size" is changed from unsigned int to int, to match what
of_get_property() expects.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>