dt-bindings: net: Add Realtek MDIO controller

dt-bindings: net: Add Realtek MDIO controller

Add dtschema for the MDIO controller found in the RTL9300 Ethernet
switch. The controller is slightly unusual in that direct MDIO
communication is not possible. We model the MDIO controller with the
MDIO buses as child nodes and the PHYs as children of the buses. The
mapping of switch port number to MDIO bus/addr requires the
ethernet-ports sibling to provide the mapping via the phy-handle
property.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patch.msgid.link/20250218195216.1034220-4-chris.packham@alliedtelesis.co.nz
Signed-off-by: Jakub Kicinski <kuba@kernel.org>