ramips: mt7620n: enable port 4 as EPHY by default

According to the datasheet the mt7620n have a fixed switch configuration
with 5 ephy (10/100) port. No RGMII configuration is possible.

Drop the mdio node as well. Without RGMII, the mdio node doesn't make any
sense

Signed-off-by: Giuseppe Lippolis <giu.lippolis@gmail.com>
[drop mdio node, enable port4 by default]
Signed-off-by: Mathias Kresin <dev@kresin.me>

target/linux/ramips/dts/mt7620n.dtsi

@@ -293,11 +293,11 @@
 
 		mediatek,switch = <&gsw>;
 
-		mdio-bus {
-			#address-cells = <1>;
-			#size-cells = <0>;
+		port@4 {
+			compatible = "mediatek,mt7620a-gsw-port", "mediatek,eth-port";
+			reg = <4>;
 
-			status = "disabled";
+			status = "okay";
 		};
 	};
 
@@ -310,7 +310,7 @@
 
 		interrupt-parent = <&intc>;
 		interrupts = <17>;
-		mediatek,port4 = "gmac";
+		mediatek,port4 = "ephy";
 	};
 
 	ehci: ehci@101c0000 {