interconnect: Add pre_aggregate() callback

Introduce an optional callback in interconnect provider drivers. It can be
used for implementing actions, that need to be executed before the actual
aggregation of the bandwidth requests has started.

The benefit of this for now is that it will significantly simplify the code
in provider drivers.

Suggested-by: Evan Green <evgreen@chromium.org>
Reviewed-by: Evan Green <evgreen@chromium.org>
Signed-off-by: Georgi Djakov <georgi.djakov@linaro.org>