[POWERPC] PS3: Storage Driver Core

Add storage driver core support for the PS3.
PS3 storage devices are a special kind of PS3 system bus device.

Signed-off-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>