Add CANFD binding documentation for Renesas RZ/G2L SoC.

Link: https://lore.kernel.org/r/20210727133022.634-2-prabhakar.mahadev-lad.rj@bp.renesas.comSigned-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>