sched: EAS/WALT: use cr_avg instead of prev_runnable_sum

WALT accounts two major statistics; CPU load and cumulative tasks
demand.

The CPU load which is account of accumulated each CPU's absolute
execution time is for CPU frequency guidance.  Whereas cumulative
tasks demand which is each CPU's instantaneous load to reflect
CPU's load at given time is for task placement decision.

Use cumulative tasks demand for cpu_util() for task placement and
introduce cpu_util_freq() for frequency guidance.

Change-Id: Id928f01dbc8cb2a617cdadc584c1f658022565c5
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>
(cherry picked from commit ee4cebd75ed7b77132c39c0093923f9ff1bcafaa)
[removed schedfreq dependency]
Signed-off-by: Quentin Perret <quentin.perret@arm.com>