arm64: dts: renesas: r8a779g0: Add RAVB nodes

Add device nodes for the Renesas Ethernet AVB (EtherAVB-IF) blocks on
the Renesas R-Car V4H (R8A779G0) SoC.

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/980e7a62d8dc3a1e2387a2d93a6296625b105506.1662715538.git.geert+renesas@glider.be