mips: dts: ralink: mt7621: add port@5 as CPU port

On MT7621AT, MT7621DAT, and MT7621ST SoCs, port 5 of the MT7530 switch is
connected to the second MAC of the SoC as a CPU port. Add the port and set
up the second MAC on the bindings. Revert PHY muxing on GB-PC1.

There's an external PHY connected to the second MAC of the SoC on GB-PC2,
therefore, disable port@5 for this device.

Signed-off-by: Arınç ÜNAL <[e-mail]>
Acked-by: Sergio Paracuellos <[e-mail]>
Signed-off-by: Thomas Bogendoerfer <[e-mail]>

arch/mips/boot/dts/ralink/mt7621-gnubee-gb-pc1.dts

···
 	status = "okay";
 };
 
-&gmac1 {
-	status = "okay";
-	phy-handle = <&ethphy4>;
-};
-
-&mdio {
-	ethphy4: ethernet-phy@4 {
-		reg = <4>;
-	};
-};
-
 &switch0 {
 	ports {
 		port@0 {
 			status = "okay";
 			label = "ethblack";
+		};
+
+		port@4 {
+			status = "okay";
+			label = "ethblue";
 		};
 	};
 };

arch/mips/boot/dts/ralink/mt7621-gnubee-gb-pc2.dts

···
 };
 
 &gmac1 {
-	status = "okay";
 	phy-mode = "rgmii-rxid";
 	phy-handle = <&ethphy5>;
+
+	fixed-link {
+		status = "disabled";
+	};
 };
 
 &mdio {
···
 		port@4 {
 			status = "okay";
 			label = "ethblue";
+		};
+
+		port@5 {
+			status = "disabled";
 		};
 	};
 };

arch/mips/boot/dts/ralink/mt7621.dtsi

···
 		gmac1: mac@1 {
 			compatible = "mediatek,eth-mac";
 			reg = <1>;
-			status = "disabled";
 			phy-mode = "rgmii";
+
+			fixed-link {
+				speed = <1000>;
+				full-duplex;
+				pause;
+			};
 		};
 
 		mdio: mdio-bus {
···
 						status = "disabled";
 						reg = <4>;
 						label = "swp4";
+					};
+
+					port@5 {
+						reg = <5>;
+						ethernet = <&gmac1>;
+						phy-mode = "rgmii";
+
+						fixed-link {
+							speed = <1000>;
+							full-duplex;
+							pause;
+						};
 					};
 
 					port@6 {