dt-bindings: can: rcar_can: document r8a77990 support

Document the support for rcar_can on R8A77990 SoC devices.
Add R8A77990 to the list of SoCs which require the "assigned-clocks"
and "assigned-clock-rates" properties.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Marc Kleine-Budde <mkl@pengutronix.de>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>