kconfig: Unify CACHE_* options

The kconfig options to configure the cache flushing framework are
currently living in the arch-specific kconfigs of ARC and X86 (32-bit)
architectures even though these are defining the same things.

Move the common symbols in one place accessible by all the architectures
and create a menu for those.

Leave the default values in the arch-specific locations.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>