arm64: dts: renesas: r8a77961: Add RAVB node

Add a device node for the Ethernet AVB (RAVB) interface on the Renesas
R-Car M3-W+ (r8a77961) SoC.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Tested-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Link: https://lore.kernel.org/r/20191216124740.16647-4-geert+renesas@glider.be