dt-bindings: net: renesas,etheravb: Add optional MDIO bus node

The Renesas Ethernet AVB bindings do not allow the MDIO bus to be
described. This has not been needed as only a single PHY is
supported and no MDIO bus properties have been needed.

Add an optional mdio node to the binding which allows the MDIO bus to be
described and allow bus properties to be set.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20240325153451.2366083-2-niklas.soderlund+renesas@ragnatech.se
Signed-off-by: Jakub Kicinski <kuba@kernel.org>