[POWERPC] Add the thread_siblings files to sysfs

This adds the /sys/devices/system/cpu/*/topology/thread_siblings
files on powerpc.  These files are already available on other
architectures.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>