dt-bindings: PCI: Add NXP Layerscape SoCs PCIe Gen4 controller

Add PCIe Gen4 controller DT bindings of NXP Layerscape SoCs.

Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>