[POWERPC] ps3: don't call ps3_system_bus_driver_register on other platforms

ps3_system_bus_driver_register is PS3 platform specific function.
On other platforms, it triggers WARN_ON in kref_get.

Signed-off-by: Kou Ishizaki <kou.ishizaki@toshiba.co.jp>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>