greybus: greybus_protocols.h Add SVC_TIMESYNC_PING

In order to verify TimeSync functionality we require a TimeSync-ping
operation where the AP can command the SVC to initiate a single strobe of
downstream TimeSync slaves, and report the FrameTime at the strobe. Ping
will only be valid after the system has transitioned to a TIMESYNC_ACTIVE
state.

In the active state each TimeSync slave will graph a single incoming SVC
strobe as a ping and will store its frame time. The AP will then gather
each last-event FrameTime for presentation to user-space in the AP and/or
further automation based on the reported FrameTimes at the SVC ping.

This patch adds the SVC ping command definition to greybus_protocols.h.

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