ARM: trusted_foundations: Support L2 cache maintenance

Implement L2 cache initialization firmware callback that should be
invoked early during boot in order to set up the required outer cache
driver's callbacks and add the callback required for L2X0 maintenance.

Partially based on work done by Michał Mirosław [1].

[1] https://www.spinics.net/lists/arm-kernel/msg594765.html



Tested-by: Robert Yang <decatf@gmail.com>
Tested-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>