powerpc: Run sbc610 USB fixup code only on the appropriate platform.

commit a969e76a (powerpc: Correct USB
support for GE Fanuc SBC610) introduced a fixup for NEC usb controllers.
This fixup should only run on GEF SBC610 boards.

Fixes Fedora bug #486511.
(https://bugzilla.redhat.com/show_bug.cgi?id=486511

)

Signed-off-by: Tony Breeds <tony@bakeyournoodle.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>