greybus: tracing: define bundle traces

Define a new gb_bundle trace point event class, used to trace events
associated with the bundle abstraction.  Define four basic trace
points for this--creation time, drop of last reference, before
adding it to its interface and when removed when its interface
is destroyed.

Signed-off-by: Alex Elder <elder@linaro.org>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>