interconnect: Add a common standard aggregate function

Currently there is one very standard aggregation method that is used by
several drivers. Let's add this as a common function, so that drivers
could just point to it, instead of copy/pasting code.

Suggested-by: Evan Green <evgreen@chromium.org>
Reviewed-by: Brian Masney <masneyb@onstation.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Reviewed-by: Evan Green <evgreen@chromium.org>
Signed-off-by: Georgi Djakov <georgi.djakov@linaro.org>