arm64: dts: renesas: r8a774c0: Add Ethernet AVB node

This patch adds the SoC specific part of the Ethernet AVB
device tree node.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>