[POWERPC] Only enable cpu hotplug via RTAS if the required firmware support is found

To support cpu hotplug on pseries we require two RTAS tokens.  The cpu
hotplug machinery should only be wired up if these tokens are found in
the device tree.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>