DEBUG: sched/fair: Fix sched_load_avg_cpu events for task_groups

The current sched_load_avg_cpu event traces the load for any cfs_rq that is
updated. This is not representative of the CPU load - instead we should only
trace this event when the cfs_rq being updated is in the root_task_group.

Change-Id: I345c2f13f6b5718cb4a89beb247f7887ce97ed6b
Signed-off-by: Brendan Jackman <brendan.jackman@arm.com>
(cherry picked from commit 1cb392e10307ba3ef7d9a602e59e54ae3b6399ad)
(cherry picked from commit 835f5ebd2f7170c87dc1387976ed1b4e46215251)
Signed-off-by: Quentin Perret <quentin.perret@arm.com>