[POWERPC] cell: Add spu shutdown method

Add a shutdown method to spu_sysdev_class to allow proper spu resource
cleanup on system shutdown.  This is needed to support kexec on the PS3
platform.

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