arm64: renesas: Move SoC Kconfig symbols to drivers/soc/renesas/

arch/arm64/Kconfig.platforms has SoC-specific Kconfig symbols for
Renesas SoCs, while other vendors have only a single Kconfig symbol.

Increase consistency with other vendors by moving the SoC-specific
Kconfig symbols to drivers/soc/renesas/Kconfig.

Increase consistency with R-Car Gen1 and Gen2 SoCs on arm32 by
introducing a family-specific Kconfig symbol for R-Car Gen3
(ARCH_RCAR_GEN3), which enables family-specific hardware features.
While so far only a single family (R-Car Gen3 and derivatives) of
Renesas arm64 SoCs is supported by Linux, this will make it easier
to add support for other SoC families later.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>