clk: renesas: Rework Kconfig and Makefile logic

The goals are to:
  - Allow precise control over and automatic selection of which
    (sub)drivers are used for which SoC (which may change in the
    future),
  - Allow adding support for new SoCs easily,
  - Allow compile-testing of all (sub)drivers,
  - Keep driver selection logic in the subsystem-specific Kconfig,
    independent from the architecture-specific Kconfig (i.e. no "select"
    from arch/arm64/Kconfig.platforms), to avoid dependencies.

This is implemented by:
  - Introducing Kconfig symbols for all drivers and sub-drivers,
  - Introducing the Kconfig symbol CLK_RENESAS, which is enabled
    automatically when building for a Renesas ARM platform, and which
    enables all required drivers without interaction of the user, based
    on SoC-specific ARCH_* symbols,
  - Allowing the user to enable any Kconfig symbol manually if
    COMPILE_TEST is enabled,
  - Using the new Kconfig symbols instead of the ARCH_* symbols to
    control compilation in the Makefile,
  - Always entering drivers/clk/renesas/ during the build.

Note that currently not all (sub)drivers are enabled for
compile-testing, as they depend on independent fixes in other
subsystems.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Acked-by: Stephen Boyd <sboyd@codeaurora.org>

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