[POWERPC] pasemi: Only call of_platform_bus_probe() on relevant platforms

Only publish of_platform devices if running on a machine that has them.

Signed-off-by: Olof Johansson <olof@lixom.net>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>