clk: versatile: Rework kconfig structure

CONFIG_COMMON_CLK_VERSATILE doesn't really do anything other than hiding
Arm Ltd reference platform clock drivers. It is both selected by the
platforms that need it and has a 'depends on' for those platforms. Let's
drop the selects and convert CONFIG_COMMON_CLK_VERSATILE into a
menuconfig entry. With this make CONFIG_ICST visible.

Move the 'select REGMAP_MMIO' to the drivers that require it (SP810 did
not).

This also has the side effect of enabling CONFIG_ICST for COMPILE_TEST
as it was not visible before.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Liviu Dudau <liviu.dudau@arm.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>