dt-bindings: PCI: rockchip: Add DT bindings for Rockchip PCIe EP driver

This patch documents the DT bindings for the Rockchip PCIe controller
when configured in EP mode.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>