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: I2f5e3ca2b64dde68d2ba4585e2d76d50c09cdf00
Signed-off-by: Brendan Jackman <brendan.jackman@arm.com>