[POWERPC] cell: register SPUs as sysdevs

SPUs are registered as system devices, exposing attributes through
sysfs. Since the sysdev includes a kref, we can remove the one in
struct spu (it isn't used at the moment anyway).

Currently only the interrupt source and numa node attributes are added.

Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>