dt-bindings: display: bridge: renesas,lvds: Document r8a77961 bindings

Document the R-Car M3-W+ (R8A77961) SoC in the R-Car LVDS encoder
bindings.

Signed-off-by: Nikita Yushchenko <nikita.yoush@cogentembedded.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>