ARM: rmobile: Turn R-Car V3U into R-Car Gen4

ARM: rmobile: Turn R-Car V3U into R-Car Gen4

Despite the name, R-Car V3U is the first member of the R-Car Gen4
family [1]. Hence reflect this in related files, select appropriate
configuration options and split DT build into its own GEN4 entry.

[1] https://www.renesas.com/us/en/products/automotive-products/automotive-system-chips-socs/r-car-v3u-best-class-r-car-v3u-asil-d-system-chip-automated-driving

Reviewed-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Signed-off-by: Hai Pham <hai.pham.ud@renesas.com>
Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
[Marek: Deduplicate DTC_FLAGS addition using RCAR_64 symbol
        Update commit message]