dt-bindings: net: snps,dwmac: Align mdio node in example with bindings

According to the bindings, the MDIO subnode should be called "mdio".
Update the example to match this.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms@kernel.org>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patch.msgid.link/308d72c2fe8e575e6e137b99743329c2d53eceea.1747121550.git.geert+renesas@glider.be


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