powerpc/pseries: Child nodes are not detached by dlpar_detach_node

Calls to dlpar_detach_node do not iterate over child nodes detaching them as
well. By iterating and detaching the child nodes we ensure that they have the
OF_DETACHED flag set and that their reference counts are decremented such that
the node will be freed from memory by of_node_release.

Signed-off-by: Tyrel Datwyler <tyreld@linux.vnet.ibm.com>
Acked-by: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>