trace:sched: Make util_avg in load_avg trace reflect PELT/WALT as used

trace:sched: Make util_avg in load_avg trace reflect PELT/WALT as used

With the ability to choose between WALT and PELT for utilisation tracking
we can have the situation where we're using WALT to make all the
decisions and reporting PELT figures in the sched_load_avg_(cpu|task)
trace points. This is not too much of an issue, but when analysing trace
it is nice to see numbers representing what the scheduler is using rather
than needing to add in additional sched_walt_* traces to figure it out.

Add reporting for both types, and make the util_avg member reflect what
will be seen from cpu or task_util functions in the scheduler.

Change-Id: I2abbd2c5fa70822096d0f3372b4c12b1c6af1590
Signed-off-by: Chris Redpath <chris.redpath@arm.com>