dt-bindings: PCI: cadence: Add DT bindings for Cadence PCIe endpoint controller

This patch documents the DT bindings for the Cadence PCIe controller
when configured in endpoint mode.
Signed-off-by: NCyrille Pitchen <cyrille.pitchen@free-electrons.com>
Signed-off-by: NLorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: NRob Herring <robh@kernel.org>

Documentation/devicetree/bindings/pci/cdns,cdns-pcie-ep.txt

+* Cadence PCIe endpoint controller
+
+Required properties:
+- compatible: Should contain "cdns,cdns-pcie-ep" to identify the IP used.
+- reg: Should contain the controller register base address and AXI interface
+  region base address respectively.
+- reg-names: Must be "reg" and "mem" respectively.
+- cdns,max-outbound-regions: Set to maximum number of outbound regions
+
+Optional properties:
+- max-functions: Maximum number of functions that can be configured (default 1).
+
+Example:
+
+pcie@fc000000 {
+	compatible = "cdns,cdns-pcie-ep";
+	reg = <0x0 0xfc000000 0x0 0x01000000>,
+	      <0x0 0x80000000 0x0 0x40000000>;
+	reg-names = "reg", "mem";
+	cdns,max-outbound-regions = <16>;
+	max-functions = /bits/ 8 <8>;
+};