dt-bindings: can: rcar_canfd: document r8a774a1 support

Document the support for rcar_canfd on R8A774A1 SoC devices.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Chris Paterson <Chris.Paterson2@renesas.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>