dt-bindings: can: renesas,rcar-canfd: Fix number of channels for R-Car V3U

According to the bindings, only two channels are supported.
However, R-Car V3U supports eight, leading to "make dtbs" failures:

        arch/arm64/boot/dts/renesas/r8a779a0-falcon.dtb: can@e6660000: Unevaluated properties are not allowed ('channel2', 'channel3', 'channel4', 'channel5', 'channel6', 'channel7' were unexpected)

Update the number of channels to 8 on R-Car V3U.
While at it, prevent adding more properties to the channel nodes, as
they must contain no other properties than a status property.

Fixes: d6254d52 ("dt-bindings: can: renesas,rcar-canfd: Document r8a779a0 support")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/all/7d41d72cd7db2e90bae069ce57dbb672f17500ae.1670431681.git.geert+renesas@glider.be


Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>