dt-bindings: net: fsl: enetc: Add bindings for the central MDIO PCIe endpoint

The on-chip PCIe root complex that integrates the ENETC ethernet
controllers also integrates a PCIe endpoint for the MDIO controller
providing for centralized control of the ENETC mdio bus.
Add bindings for this "central" MDIO Integrated PCIe Endpoint.
Signed-off-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/fsl-enetc.txt

@@ -11,7 +11,9 @@ Required properties:
 		  to parent node bindings.
 - compatible	: Should be "fsl,enetc".
 
-1) The ENETC external port is connected to a MDIO configurable phy:
+1. The ENETC external port is connected to a MDIO configurable phy
+
+1.1. Using the local ENETC Port MDIO interface
 
 In this case, the ENETC node should include a "mdio" sub-node
 that in turn should contain the "ethernet-phy" node describing the
@@ -47,8 +49,42 @@ Example:
 		};
 	};
 
-2) The ENETC port is an internal port or has a fixed-link external
-connection:
+1.2. Using the central MDIO PCIe endpoint device
+
+In this case, the mdio node should be defined as another PCIe
+endpoint node, at the same level with the ENETC port nodes.
+
+Required properties:
+
+- reg		: Specifies PCIe Device Number and Function
+		  Number of the ENETC endpoint device, according
+		  to parent node bindings.
+- compatible	: Should be "fsl,enetc-mdio".
+
+The remaining required mdio bus properties are standard, their bindings
+already defined in Documentation/devicetree/bindings/net/mdio.txt.
+
+Example:
+
+	ethernet@0,0 {
+		compatible = "fsl,enetc";
+		reg = <0x000000 0 0 0 0>;
+		phy-handle = <&sgmii_phy0>;
+		phy-connection-type = "sgmii";
+	};
+
+	mdio@0,3 {
+		compatible = "fsl,enetc-mdio";
+		reg = <0x000300 0 0 0 0>;
+		#address-cells = <1>;
+		#size-cells = <0>;
+		sgmii_phy0: ethernet-phy@2 {
+			reg = <0x2>;
+		};
+	};
+
+2. The ENETC port is an internal port or has a fixed-link external
+connection
 
 In this case, the ENETC port node defines a fixed link connection,
 as specified by Documentation/devicetree/bindings/net/fixed-link.txt.