Document DT bindings for PCIe controller found on Rockchip SoC.

Link: https://lore.kernel.org/r/20210818093406.157788-1-xxm@rock-chips.comSigned-off-by: Simon Xue <xxm@rock-chips.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>