greybus: loopback: graph round-trip time for all threads

This patch adds the ability to time the delta between all threads like this

t1 = timestmap();
thread1:gb_operation_sync();
thread2:gb_operation_sync();
t2 = timestamp();

In order to enable that behaviour without forcing an undesirable
checkpointing scheme this patch introduces a kfifo for each thread to store
the raw timestamps and calculate a time difference.

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