Document support for rcar_canfd on R8A779A0 (V3U) SoCs.

Link: https://lore.kernel.org/all/20220309162609.3726306-5-uli+renesas@fpond.euSigned-off-by: Ulrich Hecht <uli+renesas@fpond.eu>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>