[POWERPC] ps3: add ps3 platform system bus support

Adds a PS3 system bus driver.  This system bus is a virtual bus used to present
the PS3 system devices in the LDM.

Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>