sched: introduce per CPU sched_load_boost knob

Introduce a new knob sched_load_boost that allows userspace to
provide additional CPU capacity margin to work running on CPUs.  The
applied margin is linearly proportional to the actual load on the CPU
and sched_load_boost.  The applied margin is not affected by the max
capacity of the CPUs in the system.

Userspace could increase this for exceedingly sporadic workloads in
case the existing margin isn't sufficient.

Also, while at it, refactor cpu_util_freq().

Change-Id: I07379aa3c5d9e670f67886323e2e32225a1bdef9
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>