greybus: timesync: Add debugfs entry to display frame-ping in ktime

This patch makes a debugfs entry in
/sys/kernel/debug/greybus/X-svc/frame-ktime that generates a TimeSync ping
event to the system and then subsequently presents that data to user-space
as a ktime/timespec clock-monotonic value rather than as a raw frame-time,
to aid humans in debugging and understanding frame-time and to provide an
example of the converting a frame-time to timespec/ktime to other
developers.

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