dt-bindings: can: rcar_canfd: Convert to json-schema

Convert the Renesas R-Car CAN FD Controller Device Tree binding
documentation to json-schema.

Document missing properties.
The CANFD clock needs to be configured for the maximum frequency on
R-Car V3M and V3H, too.
Update the example to match reality.

Link: https://lore.kernel.org/r/905134c87f72e2d8e37c309e0ce28ecd7d4f3992.1620323639.git.geert+renesas@glider.be


Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Ulrich Hecht <uli+renesas@fpond.eu>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>