sched: add per CPU sched_load_boost knob

Commit '858d5751bcf7b6a ("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 in
msm-4.9. Userspace could increase this for exceedingly sporadic workloads
in case the existing margin isn't sufficient.

While porting it, sched_load_boost support is ported but missed adding
procfs interface. Add it now to let user space configure the knob.

Change-Id: I4277c63089f6bb5227d257ab7484d50c42827c86
Signed-off-by: Lingutla Chandrasekhar <clingutla@codeaurora.org>