airoha: add GDM1 sub-node into EN7523 ethernet controller node

This is required to make ethernet working after PCS support changes

Signed-off-by: Mikhail Kshevetskiy <mikhail.kshevetskiy@iopsys.eu>

arch/arm/dts/en7523-evb-u-boot.dtsi

@@ -9,3 +9,7 @@
 };
 
 #include "en7523-u-boot.dtsi"
+
+&gdm1 {
+	status = "okay";
+};

arch/arm/dts/en7523-u-boot.dtsi

@@ -37,6 +37,19 @@
 			 <&scu EN7523_HSI_MAC_RST>;
 		reset-names = "fe", "pdma", "qdma",
 			      "hsi0-mac", "hsi1-mac", "hsi-mac";
+
+		gdm1: ethernet@1 {
+			compatible = "airoha,eth-mac";
+			reg = <1>;
+			phy-mode = "internal";
+			status = "disabled";
+
+			fixed-link {
+				speed = <10000>;
+				full-duplex;
+				pause;
+			};
+		};
 	};
 
 	switch: switch@1fb58000 {