greybus: tracing: add module traces

Define a new gb_module trace point event class, used to trace events
associated with the module abstraction.  Define four basic trace
points for this--creation time, drop of last reference, before
registring interfaces and after de-registering them.

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