greybus: loopback: capture and present firmware supplied latencies

In order to provide deep inspection of the greybus/UniPro system
instrumentation of

1. APBridge's view of UniPro latency
2. GPBridge's view of internal firmware-only latency

have both been added and reported to the AP in the transfer loopback
response header. When this data are present we latch and average it over
the number of requested cycles, presenting it to user-space via sysfs.

This patch adds the following sysfs entries for each loopback CPort

- apbridge_unipro_latency_avg_con
- apbridge_unipro_latency_max_con
- apbridge_unipro_latency_min_con
- gpbridge_firmware_latency_avg_con
- gpbridge_firmware_latency_max_con
- gpbridge_firmware_latency_min_con

and the following sysfs entries representing the average values across all
available CPorts

- apbridge_unipro_latency_avg_dev
- apbridge_unipro_latency_max_dev
- apbridge_unipro_latency_min_dev
- gpbridge_firmware_latency_avg_dev
- gpbridge_firmware_latency_max_dev
- gpbridge_firmware_latency_min_dev

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