greybus: loopback: support synchronized tests over multiple cports

The loopback code as currently implemented allows free running threads to
blast data to cports in isolation, however no overall stastics are gathered
for the aggregate throughput to a given module.

This patch moves derivation of stastics for all cports to one structure
so that 1 to n cports will report their overall stastics in one place.
Later patches update the sysfs/debugfs accessor functions to provide the
per-module and per-connection data separately.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>