clk: Allow the common clk framework to be selectable

Enable build testing and configuration control of the common clk
framework so that more code coverage and testing can be done on the
common clk framework across various architectures. This also nicely
removes the requirement that architectures must select the framework
when they don't use it in architecture code.

There's one snag with doing this, and that's making sure that randconfig
builds don't select this option when some architecture or platform
implements 'struct clk' outside of the common clk framework. Introduce a
new config option 'HAVE_LEGACY_CLK' to indicate those platforms that
haven't migrated to the common clk framework and therefore shouldn't be
allowed to select this new config option. Also add a note that we hope
one day to remove this config entirely.

Based on a patch by Mark Brown <broonie@kernel.org>.

Cc: Mark Brown <broonie@kernel.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Mark Salter <msalter@redhat.com>
Cc: Aurelien Jacquiot <jacquiot.aurelien@gmail.com>
Cc: Jiaxun Yang <jiaxun.yang@flygoat.com>
Cc: Guan Xuetao <gxt@pku.edu.cn>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Cc: Rich Felker <dalias@libc.org>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: <linux-mips@vger.kernel.org>
Cc: <linux-c6x-dev@linux-c6x.org>
Cc: <linux-m68k@lists.linux-m68k.org>
Cc: <linux-arm-kernel@lists.infradead.org>
Cc: <linux-sh@vger.kernel.org>
Link: https://lore.kernel.org/r/1470915049-15249-1-git-send-email-broonie@kernel.org


Signed-off-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lkml.kernel.org/r/20200409064416.83340-8-sboyd@kernel.org


Reviewed-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>