dt-bindings: PCI: rockchip: Add DT bindings for Rockchip PCIe controller

Add a binding that describes the Rockchip PCIe controller found on Rockchip
SoCs PCIe interface.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Brian Norris <briannorris@chromium.org>
Acked-by: Rob Herring <robh@kernel.org>