can: rcar_canfd: Add external_clk variable to struct rcar_canfd_hw_info

can: rcar_canfd: Add external_clk variable to struct rcar_canfd_hw_info

All existing SoCs support an external clock, but RZ/G3E has only internal
clocks. Add external_clk variable to struct rcar_canfd_hw_info to handle
this difference.

Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Link: https://patch.msgid.link/20250417054320.14100-18-biju.das.jz@bp.renesas.com
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>