ARM: cache_l2x0: Set outer_cache resume

Set the outer_cache.resume function during l2x0_init and declare a static
inline void version to allow compilation when OUTER_CACHE is not defined.
The outer_cache functions are used for L2 cache management in chips that
have an external cache controller. The outer_cache.resume re-enables the
L2 cache controller and is called when exiting power collapse.
This change is needed to allow drivers to call outer_cache API's instead
of directly using the APIs defined in cache-l2x0.c and thus avoid having
to use #ifdef macros in the code.

Change-Id: Ifc7d25f905e3248f864c9eaf019436968e77b8a2
Signed-off-by: Girish Mahadevan <girishm@codeaurora.org>