fwk_multi_thread: Log allocated event instead of submitted event

Currently put_event() logs information about the 'submitted event'
in its parameters. However it is more useful to show the
information about the 'allocated_event' which actually gets
enqueued by the framework.

Hence, replace the 'submitted event' with the 'allocated event' in
call to FWK_LOG_TRACE.

Change-Id: I4fa3971da782b00b26759f8efa810a96e51e81d9
Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>