cache: Introduce external cache controller system support

The cache API currently shipped in Zephyr is assuming that the cache
controller is always on-core thus managed at the arch level. This is not
always the case because many SoCs rely on external cache controllers as
a peripheral external to the core (for example PL310 cache controller
and the L2Cxxx family). In some cases you also want a single driver to
control a whole set of cache controllers.

Rework the cache code introducing support for external cache
controllers.

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