Adds CAN FD controller node for r8a7796.

Based on a patch for r8a7795 by Ramesh Shanmugasundaram.
Signed-off-by: Chris Paterson <chris.paterson2@renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Marc Kleine-Budde <mkl@pengutronix.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>