sync: Add a "details" callback for sync points

Allow drivers to add a callback for expanded details about a sync
point.  This provides for a much richer debug experience than can
be provided by the simpler callbacks.

Change-Id: Ic0dedbad19fddc2f9b753d886994247e8025d6dc
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>