drivers: mdio: add stm32 mdio support

MDIO is part of the ETH IP, but some phy chip may need a
specific phy driver to set up certain vendor registers enabling
particular features.

Add support for stm32 mdio access.

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

dts/bindings/mdio/st,stm32-mdio.yaml

@@ -0,0 +1,24 @@
+# Copyright (c) 2024 BayLibre, SAS
+# Copyright (c) 2024 Analog Devices Inc.
+# SPDX-License-Identifier: Apache-2.0
+
+description: ST MDIO Features
+
+compatible: "st,stm32-mdio"
+
+include: [mdio-controller.yaml, pinctrl-device.yaml]
+
+properties:
+  "#address-cells":
+    required: true
+    const: 1
+
+  "#size-cells":
+    required: true
+    const: 0
+
+  pinctrl-0:
+    required: true
+
+  pinctrl-names:
+    required: true