[POWERPC] Hook of_platform_bus_probe with cell

Hook up of_platform_bus_probe with the cell platform in order to publish
the non-PCI devices in the device-tree of cell blades as of_platform_device(s)

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>