dt-bindings: can: rcar_can: document r8a774b1 support

Document RZ/G2N (r8a774b1) SoC specific bindings.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1570717560-7431-2-git-send-email-fabrizio.castro@bp.renesas.com


Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>