dt-bindings: renesas,rcar-dmac: R-Car V3U is R-Car Gen4

Despite the name, R-Car V3U is the first member of the R-Car Gen4
family.  Hence move its compatible value to the R-Car Gen4 section.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/e6e4cf701f3a43b061b9c3f7f0adc4d6addd4722.1651497024.git.geert+renesas@glider.be


Signed-off-by: Vinod Koul <vkoul@kernel.org>