Switch from defining the PHY inside the AVB node itself and create a
dedicated MDIO node for AVB0, the only AVB describing a PHY.  This is
needed as adding PHYs to AVB1 and AVB2 will require setting MDIO bus
parameters and thus requires a dedicated node.
Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20240413141806.300989-2-niklas.soderlund+renesas@ragnatech.seSigned-off-by: Geert Uytterhoeven <geert+renesas@glider.be>