trace/sched: add rq utilization signal for WALT

It is useful to be able to check current capacity against rq utilization
signal generated by WALT (to check how a cpufreq governor is behaving
for example).

Add rq utilization signal (same scale as capacity) to the walt_update_
task_ravg tracepoint.

Change-Id: I9aae3884a741d23ac494bef80d2303f107f135ce
Signed-off-by: Juri Lelli <juri.lelli@arm.com>