dt-bindings: can: renesas,rcar-canfd: document r8a77961 support

This patch adds documentation for the r8a77961 to the
renesas,rcar-canfd binding.

Link: https://lore.kernel.org/all/20220401153743.77871-1-wsa+renesas@sang-engineering.com
Cc: devicetree@vger.kernel.org
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>