greybus: tracing: define connection traces

Define a new gb_connection trace point event class, used to trace
events associated with the connection abstraction.  Define four basic
trace events for this--creation, drop of last reference, and when
adding or dropping any other reference.  There are certainly
more events that we might want to add, but aim is to just get the
basic framework in place.

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