dt-bindings: can: rcar_canfd: document r8a77990 support

Document the support for rcar_canfd on R8A77990 SoC devices.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Acked-by: David S. Miller <davem@davemloft.net>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>