powerpc/ps3: Add a sub-match id to ps3_system_bus

Add sub match id for ps3 system bus so that two different system bus
devices can be connected to a shared device.

Signed-off-by: Masakazu Mokuno <mokuno@sm.sony.co.jp>
Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>