PCI: xilinx: Add missing of_node_put()

The call to of_get_next_child() returns a node pointer with refcount
incremented thus it must be explicitly decremented here after the last
usage.

Fixes: 8961def5 ("PCI: xilinx: Add Xilinx AXI PCIe Host Bridge IP driver")
Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
[lorenzo.pieralisi@arm.com: reworked commit log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>