dt-bindings: net: Document support for Airoha AN7583 MDIO Controller

Airoha AN7583 SoC have 3 different MDIO Controller. One comes from
the intergated Switch based on MT7530. The other 2 live under the SCU
register and expose 2 dedicated MDIO controller.

Document the schema for the 2 dedicated MDIO controller.
Each MDIO controller can be independently reset with the SoC reset line.
Each MDIO controller have a dedicated clock configured to 2.5MHz by
default to follow MDIO bus IEEE 802.3 standard.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>