[POWERPC] PS3: System-bus modinfo attribute

Add modinfo attribute to ps3_system_bus devices.  Also make them all
children of the same ps3_system_bus 'device' so they appear in a
corresponding subdirectory under /sys/devices.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>