Add support for USB on PSC3 for the mpc5200

Support USB on PSC3 on the mpc5200.  Before this patch, enabling USB support
would reconfigure PSC4 and PSC5 to USB.  The mpc5200 does not support USB
enabled on both the standard USB port and PSC3.  This patch masks the
appropriate bits when enabling USB.

Signed-off-by: Eric Millbrandt <emillbrandt@dekaresearch.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Remy Bohmer <linux@bohmer.net>