dts: arm: st: add mdio node for h5 and h7

Add mdio node for h5 and h7 series.

Since MDIO registers are part of the same ETH hw IP, keeping mdio
node just as a child of mac/eth, cannot see as appropriate to assign
an adddress to it.

Signed-off-by: Angelo Dureghello <adureghello@baylibre.com>

Author: spectrum70

dts/arm/st/h5/stm32h5.dtsi

@@ -495,6 +495,13 @@
 				 <&rcc STM32_CLOCK_BUS_AHB1 0x00100000>,
 				 <&rcc STM32_CLOCK_BUS_AHB1 0x00200000>;
 			status = "disabled";
+
+			mdio: mdio {
+				compatible = "st,stm32-mdio";
+				#address-cells = <1>;
+				#size-cells = <0>;
+				status = "disabled";
+			};
 		};
 
 		gpdma1: dma@40020000 {

dts/arm/st/h7/stm32h7.dtsi

@@ -1015,6 +1015,13 @@
 				 <&rcc STM32_CLOCK_BUS_AHB1 0x00010000>,
 				 <&rcc STM32_CLOCK_BUS_AHB1 0x00020000>;
 			status = "disabled";
+
+			mdio: mdio {
+				compatible = "st,stm32-mdio";
+				#address-cells = <1>;
+				#size-cells = <0>;
+				status = "disabled";
+			};
 		};
 
 		fmc: memory-controller@52004000 {