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>