net: dsa: LAN9303: add MDIO managed mode support

When the LAN9303 device is in MDIO manged mode, all register accesses must
be done via MDIO.

Please note: this code is compile time tested only due to the absence of such
configured hardware. It is based on a patch from Stefan Roese from 2014.

Signed-off-by: Juergen Borleis <jbe@pengutronix.de>
CC: devicetree@vger.kernel.org
CC: robh+dt@kernel.org
CC: mark.rutland@arm.com
CC: sr@denx.de
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/dsa/lan9303.txt

@@ -3,7 +3,10 @@ SMSC/MicroChip LAN9303 three port ethernet switch
 
 Required properties:
 
-- compatible: should be "smsc,lan9303-i2c"
+- compatible: should be
+  - "smsc,lan9303-i2c" for I2C managed mode
+    or
+  - "smsc,lan9303-mdio" for mdio managed mode
 
 Optional properties:
 
@@ -60,3 +63,43 @@ I2C managed mode:
 			};
 		};
 	};
+
+MDIO managed mode:
+
+	master: masterdevice@X {
+		status = "okay";
+		phy-handle = <&switch>;
+
+		mdio {
+			#address-cells = <1>;
+			#size-cells = <0>;
+
+			switch: switch-phy@0 {
+				compatible = "smsc,lan9303-mdio";
+				reg = <0>;
+				reset-gpios = <&gpio7 6 GPIO_ACTIVE_LOW>;
+				reset-duration = <100>;
+
+				ports {
+					#address-cells = <1>;
+					#size-cells = <0>;
+
+					port@0 {
+						reg = <0>;
+						label = "cpu";
+						ethernet = <&master>;
+					};
+
+					port@1 { /* external port 1 */
+						reg = <1>;
+						label = "lan1;
+					};
+
+					port@2 { /* external port 2 */
+						reg = <2>;
+						label = "lan2";
+					};
+				};
+			};
+		};
+	};