arm64: dts: renesas: white-hawk-cpu: Add Ethernet support

Describe the wiring of the first Ethernet AVB instance to the Micrel
KSZ9031RNXVB PHY.

Based on a larger patch in the BSP by Takeshi Kihara.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/50a31bc8267ab4c90bff27ef3aca1169f8ebc7ae.1662715538.git.geert+renesas@glider.be