Add the documentation for the Device Tree binding for the layerscape PCIe
controller with EP mode.

Signed-off-by: Xiaowei Bao <xiaowei.bao@nxp.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Minghuan Lian <minghuan.lian@nxp.com>
Reviewed-by: Zhiqiang Hou <zhiqiang.hou@nxp.com>
Reviewed-by: Rob Herring <robh+dt@kernel.org>