greybus: loopback: add reserved fields to track firmware latencies

The greybus specification has been extended to include two new reserved
fields, which the implementation is using to track internal firmware
latencies. This change adds the appropriate fields to the corresponding
kernel header.

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