dt-bindings: pci: layerscape-pci: add compatible strings "fsl, ls1028a-pcie"

Add the PCIe compatible string for LS1028A.

Signed-off-by: Xiaowei Bao <xiaowei.bao@nxp.com>
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>
Reviewed-by: Andrew Murray <andrew.murray@arm.com>