ramips: mt7620n: add mdio node and disable port4 by default

Revert the changes I applied to aa5014d ("ramips: mt7620n: enable
port 4 as EPHY by default").

The driver expects a node mdio-bus to be present, regardless of the
actual node status. If the node is missing the driver fails to load  with

  mtk_soc_eth 10100000.ethernet: no mdio-bus child node found

Disable port4 by default again. If the port is enabled but not present, a
"invalid port id 4" warning is shown during boot.

Fixes: FS#1428

Signed-off-by: Mathias Kresin <dev@kresin.me>

target/linux/ramips/dts/mt7620n.dtsi

@@ -293,11 +293,18 @@
 
 		mediatek,switch = <&gsw>;
 
+		mdio-bus {
+			#address-cells = <1>;
+			#size-cells = <0>;
+
+			status = "disabled";
+		};
+
 		port@4 {
 			compatible = "mediatek,mt7620a-gsw-port", "mediatek,eth-port";
 			reg = <4>;
 
-			status = "okay";
+			status = "disabled";
 		};
 	};