arm64: dts: mt7986: add port@5 as CPU port

On MediaTek MT7986b RFB and MediaTek MT7986a RFB, port 5 of the MT7531
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.
Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Link: https://lore.kernel.org/r/20240219-for-mediatek-v1-2-7078f23eab82@arinc9.comSigned-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>

arch/arm64/boot/dts/mediatek/mt7986a-rfb.dts

@@ -65,6 +65,18 @@ fixed-link {
 		};
 	};
 
+	gmac1: mac@1 {
+		compatible = "mediatek,eth-mac";
+		reg = <1>;
+		phy-mode = "rgmii";
+
+		fixed-link {
+			speed = <1000>;
+			full-duplex;
+			pause;
+		};
+	};
+
 	mdio: mdio-bus {
 		#address-cells = <1>;
 		#size-cells = <0>;
@@ -288,6 +300,18 @@ port@4 {
 			label = "lan4";
 		};
 
+		port@5 {
+			reg = <5>;
+			ethernet = <&gmac1>;
+			phy-mode = "rgmii";
+
+			fixed-link {
+				speed = <1000>;
+				full-duplex;
+				pause;
+			};
+		};
+
 		port@6 {
 			reg = <6>;
 			label = "cpu";

arch/arm64/boot/dts/mediatek/mt7986b-rfb.dts

@@ -45,6 +45,18 @@ fixed-link {
 		};
 	};
 
+	gmac1: mac@1 {
+		compatible = "mediatek,eth-mac";
+		reg = <1>;
+		phy-mode = "rgmii";
+
+		fixed-link {
+			speed = <1000>;
+			full-duplex;
+			pause;
+		};
+	};
+
 	mdio: mdio-bus {
 		#address-cells = <1>;
 		#size-cells = <0>;
@@ -83,6 +95,18 @@ port@4 {
 					label = "lan4";
 				};
 
+				port@5 {
+					reg = <5>;
+					ethernet = <&gmac1>;
+					phy-mode = "rgmii";
+
+					fixed-link {
+						speed = <1000>;
+						full-duplex;
+						pause;
+					};
+				};
+
 				port@6 {
 					reg = <6>;
 					label = "cpu";