Add CAN binding documentation for Renesas RZ/N1 SoC.

The SJA1000 CAN controller on RZ/N1 SoC has some differences compared
to others like it has no clock divider register (CDR) support and it has
no HW loopback (HW doesn't see tx messages on rx), so introduced a new
compatible 'renesas,rzn1-sja1000' to handle these differences.

Link: https://lore.kernel.org/all/20220710115248.190280-3-biju.das.jz@bp.renesas.comSigned-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>