[POWERPC] CPM: Change from fsl,brg-frequency to brg/clock-frequency

As suggested by David Gibson, now that we have a separate node
for the baud rate generators, it's better to use the standard
clock-frequency property than a cpm-node-level fsl,brg-frequency
property.

This patch updates existing places where fsl,brg-frequency is
used.

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