dt-bindings: net: ravb: Document r8a77961 support

Document support for the Ethernet AVB interface in the Renesas R-Car
M3-W+ (R8A77961) SoC.

Update all references to R-Car M3-W from "r8a7796" to "r8a77960", to
avoid confusion between R-Car M3-W (R8A77960) and M3-W+.

No driver update is needed.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Rob Herring <robh@kernel.org>