soc: renesas: Introduce ARCH_RCAR_GEN4

Currently, all Kconfig symbols for R-Car Gen4 SoCs select the
ARCH_RCAR_GEN3 SoC family symbol, which might confuse the casual reader.

Fix this by introducing a new SoC family symbol for R-Car Gen4 SoCs.
For now this just selects ARCH_RCAR_GEN3, to avoid duplication, and to
relax dependencies.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/r/a283913ba7a39ec2dae0ebf9fc2b06096f009511.1706194617.git.geert+renesas@glider.be