[core/sched/cbs] Keep track of Round start/end timestamps

The round start, as well as the _expected_ round end time, are now tracked
by RQ structure. These values could be useful to debug and trace the usage
of HRTimer by the CBS code.

Signed-off-by: Patrick Bellasi <derkling@gmail.com>
[core/sched/cbs] Trace usage of HRTimer for burst preemption

Once a burst is completed, a flag report if it has been preempted by a
scheduled HRTimer or just by timer tick.

Signed-off-by: Patrick Bellasi <derkling@gmail.com>
[core/sched/cbs] Bursts/Rounds FTrace of start and stop/end timestamps

The start and stop/end timestamps of each burst/round is traced to better
support debugging of HRTimer usage.

Signed-off-by: Patrick Bellasi <derkling@gmail.com>