greybus: loopback: add tracker variables to hold firmware timestamps

This patch adds tracker variables to hold the incoming firmware derived
timestamps where apbridge_latency_ts will contain the APBridge's view of
the UniPro turn-around time and gpbridge_latency_ts will contain the
GPBridge's view of it's own internal latency. Both values are reported
in microseconds.

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