greybus: loopback: add ability to graph greybus latency

This patch adds the ability to graph the latency of the overhead introduced
by the greybus stack in the roundtrip time AP->Module->AP.

Since this is a small number it is reported in nanoseconds, not
mircoseconds. This data can also be derived with tracepoints but it's being
provided in this format to export to CSV file more easily than with
tracepoints.

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