arm64: dts: rockchip: add basic mdio node to px30

arm64: dts: rockchip: add basic mdio node to px30

Using snps,reset-* properties for handling the phy-reset is deprecated
and instead a real phy node should be defined that then contains the
reset-gpios handling.

To facilitate this, add the core mdio node under the px30's gmac, similar
to how the other Rockchip socs already do this.

Signed-off-by: Heiko Stuebner <heiko.stuebner@cherry.de>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Link: https://lore.kernel.org/r/20250514150745.2437804-2-heiko@sntech.de
Signed-off-by: Heiko Stuebner <heiko@sntech.de>