dt-bindings: iommu: renesas,ipmmu-vmsa: 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>
Acked-by: Joerg Roedel <jroedel@suse.de>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
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v2:
  - Add Acked-by, Reviewed-by,
  - Add blank lines to improve readability.