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>
[ upstream commit: ede4837a504ca7e5811217060aa8300b8d0cf7f2 ]

(cherry picked from commit cb54a264ecdb9e95b1529e4542e157cb9acded30)

Reviewed-by: Sumit Garg <sumit.garg@oss.qualcomm.com>