dt-bindings: add DT binding for the layerscape PCIe controller with EP mode

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>

Documentation/devicetree/bindings/pci/layerscape-pci.txt

@@ -13,6 +13,7 @@ information.
 
 Required properties:
 - compatible: should contain the platform identifier such as:
+  RC mode:
         "fsl,ls1021a-pcie"
         "fsl,ls2080a-pcie", "fsl,ls2085a-pcie"
         "fsl,ls2088a-pcie"
@@ -20,6 +21,8 @@ Required properties:
         "fsl,ls1046a-pcie"
         "fsl,ls1043a-pcie"
         "fsl,ls1012a-pcie"
+  EP mode:
+	"fsl,ls1046a-pcie-ep", "fsl,ls-pcie-ep"
 - reg: base addresses and lengths of the PCIe controller register blocks.
 - interrupts: A list of interrupt outputs of the controller. Must contain an
   entry for each entry in the interrupt-names property.