dt-bindings: i2c: renesas,rcar-i2c: R-Car V3U is R-Car Gen4

Despite the name, R-Car V3U is the first member of the R-Car Gen4
family.  I2C on R-Car V3U also supports some extra features (e.g. Slave
Clock Stretch Select), which are supported by other R-Car Gen4 SoCs, but
not by any other R-Car Gen3 SoC.

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>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>