mpc86xx: Double local bus clock divider

The local bus clock divider should be doubled for both 8610 and 8641.

Signed-off-by: Trent Piepho <tpiepho@freescale.com>
Acked-by: Kumar Gala <galak@kernel.crashing.org>
Acked-by: Jon Loeliger <jdl@freescale.com>