sched: make sched_cpu_high_irqload a runtime tunable

It may be desirable to be able to alter the scehd_cpu_high_irqload
setting easily, so make it a runtime tunable value.

Change-Id: I832030eec2aafa101f0f435a4fd2d401d447880d
Signed-off-by: Steve Muckle <smuckle@codeaurora.org>