MIPS: pci-rt2880: set pci controller of_node

Set the PCI controller of_node such that PCI devices can be
instantiated via device tree.

Signed-off-by: Tobias Wolf <dev-NTEO@vplace.de>
Signed-off-by: Mathias Kresin <dev@kresin.me>
Acked-by: John Crispin <john@phrozen.org>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/20423/
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org