[POWERPC] 86xx: MPC8641 HPCN - call of_platform_bus_probe()

Call of_platform_bus_probe() on the MPC8641 HPCN, similar to what is
done for other platforms.

Signed-off-by: Wade Farnsworth <wfarnsworth@mvista.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>