dt-bindings: net: renesas,ethertsn: Create child-node for MDIO bus

The bindings for Renesas Ethernet TSN was just merged in v6.9 and the
design for the bindings followed that of other Renesas Ethernet drivers
and thus did not force a child-node for the MDIO bus. As there
are no upstream drivers or users of this binding yet take the
opportunity to correct this and force the usage of a child-node for the
MDIO bus.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20240330131228.1541227-1-niklas.soderlund+renesas@ragnatech.se


Signed-off-by: Jakub Kicinski <kuba@kernel.org>