dt-bindings: net: renesas,etheravb: Update example to match reality

- Add missing clock-names property,
  - Add example compatible values for PHY subnode.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: David S. Miller <davem@davemloft.net>