cpufreq/schedutil: use boosted_cpu_util for PELT to match WALT

When using WALT we always used boosted cpu util for OPP selection.
This is the primary purpose for boosted cpu util, but we hadn't
changed the PELT utilization check to do the same thing.

Fix that here.

Change-Id: Id5ffb26eac23b25fe754255221f6d21b8cededfd
Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
Signed-off-by: Chris Redpath <chris.redpath@arm.com>