irqchip/gic-v3: Fix ppi-partitions lookup

Fix child-node lookup during initialisation, which ended up searching
the whole device tree depth-first starting at the parent rather than
just matching on its children.

To make things worse, the parent gic node was prematurely freed, while
the ppi-partitions node was leaked.

Fixes: e3825ba1 ("irqchip/gic-v3: Add support for partitioned PPIs")
Cc: stable <stable@vger.kernel.org>     # 4.7
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>