sched: avoid CPUs with high irq activity

CPUs with significant IRQ activity will not be able to serve tasks
quickly. Avoid them if possible by disqualifying such CPUs from
being recognized as mostly idle.

Change-Id: I2c09272a4f259f0283b272455147d288fce11982
Signed-off-by: Steve Muckle <smuckle@codeaurora.org>