riscv: dts: sophgo: Add mdio multiplexer device for cv18xx

Add DT device node of mdio multiplexer device for cv18xx SoC.

Link: https://lore.kernel.org/r/20250703021600.125550-3-inochiama@gmail.com
Signed-off-by: Inochi Amaoto <inochiama@gmail.com>
Signed-off-by: Chen Wang <unicorn_wang@outlook.com>
Signed-off-by: Chen Wang <wangchen20@iscas.ac.cn>

Author: inochisa

arch/riscv/boot/dts/sophgo/cv180x.dtsi

@@ -31,6 +31,33 @@
 			#reset-cells = <1>;
 		};
 
+		mdio: mdio@3009800 {
+			compatible = "mdio-mux-mmioreg", "mdio-mux";
+			reg = <0x3009800 0x4>;
+			#address-cells = <1>;
+			#size-cells = <0>;
+			mdio-parent-bus = <&gmac0_mdio>;
+			mux-mask = <0x80>;
+			status = "disabled";
+
+			internal_mdio: mdio@0 {
+				#address-cells = <1>;
+				#size-cells = <0>;
+				reg = <0>;
+
+				internal_ephy: phy@0 {
+					compatible = "ethernet-phy-ieee802.3-c22";
+					reg = <1>;
+				};
+			};
+
+			external_mdio: mdio@80 {
+				#address-cells = <1>;
+				#size-cells = <0>;
+				reg = <0x80>;
+			};
+		};
+
 		gpio0: gpio@3020000 {
 			compatible = "snps,dw-apb-gpio";
 			reg = <0x3020000 0x1000>;
@@ -196,6 +223,8 @@
 			clock-names = "stmmaceth", "ptp_ref";
 			interrupts = <SOC_PERIPHERAL_IRQ(15) IRQ_TYPE_LEVEL_HIGH>;
 			interrupt-names = "macirq";
+			phy-handle = <&internal_ephy>;
+			phy-mode = "internal";
 			resets = <&rst RST_ETH0>;
 			reset-names = "stmmaceth";
 			rx-fifo-depth = <8192>;