MPC8548: dts: Added PCIe DT node

MPC8548 integrated a PCIe controllers, which is compatible with
the PCI Express™ Base Specification, Revision 1.0a, and this
patch is to add DT node for the PCIe controller.

Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Prabhakar Kushwaha <prabhakar.kushwaha@nxp.com>