ANDROID: sched/cpufreq: make schedutil use WALT signal

If WALT is available and enabled, make schedutil governor use its
utilization signal.

Change-Id: I92bc37989447a76616e9bcc4e9e8616774fb9925
Signed-off-by: Juri Lelli <juri.lelli@arm.com>
[we need to use boosted_cpu_util for schedutil, so make it
 not static]
Signed-off-by: Chris Redpath <chris.redpath@arm.com>
Signed-off-by: Quentin Perret <quentin.perret@arm.com>