[POWERPC] sysfs: add support for adding/removing spu sysfs attributes

This patch adds two functions to create and remove sysfs attributes and
attribute_group to all cpus.  That allows to register sysfs attributes in
a subdirectory like: /sys/devices/system/cpu/cpuX/group_name/what_ever
This will be used by cbe_thermal to group all attributes dealing with
thermal support in one directory.

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