HMP: Implement task packing for small tasks in HMP systems

If we wake up a task on a little CPU, fill CPUs rather than
spread. Adds 2 new files to sys/kernel/hmp to control packing
behaviour.

packing_enable: task packing enabled (1) or disabled (0)
packing_limit: Runqueues will be filled up to this load ratio.

This functionality is disabled by default on TC2 as it lacks per-cpu
power gating so packing small tasks there doesn't make sense.

Signed-off-by: Chris Redpath <chris.redpath@arm.com>
Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>
Signed-off-by: Jon Medhurst <tixy@linaro.org>

Conflicts:

	arch/arm/Kconfig
	kernel/sched/fair.c