powerpc/86xx: Run sbc310 USB fixup code only on the appropriate platform.

Patch to limit NEC fixup to SBC310, following similar patch to SBC610 by
Tony Breeds: 368a1211

Signed-off-by: Martyn Welch <martyn.welch@gefanuc.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>