sched: track soft/hard irqload per-RQ with decaying avg

The scheduler currently ignores irq activity when deciding which
CPUs to place tasks on. If a CPU is getting hammered with IRQ activity
but has no tasks it will look attractive to the scheduler as it will
not be in a low power mode.

Track irqload with a decaying average. This quantity can be used
in the task placement logic to avoid CPUs which are under high
irqload. The decay factor is 3/4. Note that with this algorithm the
tracked irqload quantity will be higher than the actual irq time
observed in any single window. Some sample outcomes with steady
irqloads per 10ms window and the 3/4 decay factor (irqload of 10 is
used as a threshold in a subsequent patch):

irqload per window        load value asymptote      # windows to > 10
2ms			  8			    n/a
3ms			  12			    7
4ms			  16			    4
5ms			  20			    3

Of course irqload will not be constant in each window, these are just
given as simple examples.

Change-Id: I9dba049f5dfdcecc04339f727c8dd4ff554e01a5
Signed-off-by: Steve Muckle <smuckle@codeaurora.org>