DT: net: can: rcar_canfd: document R8A77980 bindings

Document the R-Car V3H (R8A77980) SoC support in the R-Car CAN-FD bindings.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>