greybus: connection: add a timestamp kfifo to track connection handoff

For the ES2 test activity it may be beneficial to have a performance
metric that doesn't include any of the greybus stack malloc, workqueues
etc. In order to faciltate, this patch adds a simple kfifo structure to
hold two timestamp values. One timestamp will represent the last reasonable
point a greybus outbound timestamp can be taken, the other timestamp will
represent the first reasonable point an inbound timestamp can be taken. In
order to facilitate this model, tracking the timestamps in the connection
structure appears to be the best place to keep store of this data.

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