[POWERPC] cbe_thermal: Fix initialization of sysfs attribute_group

This patch adds NULL to the initialization of the attribute_groups.
The spu_attributes and ppe_attributes arrays are arrays of pointers
that need to be terminated with a NULL entry.

Signed-off-by: Christian Krafft <krafft@de.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>