[POWERPC] PS3: Move system bus to platform directory

Move the PS3 system bus routines from drivers/ps3 to
arch/powerpc/platforms/ps3.

Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>