sched: Task placement for heterogeneous systems based on task load-tracking

sched: Task placement for heterogeneous systems based on task load-tracking

This patch introduces the basic SCHED_HMP infrastructure. Each class of
cpus is represented by a hmp_domain and tasks will only be moved between
these domains when their load profiles suggest it is beneficial.

SCHED_HMP relies heavily on the task load-tracking introduced in Paul
Turners fair group scheduling patch set:

<https://lkml.org/lkml/2012/8/23/267>

SCHED_HMP requires that the platform implements arch_get_hmp_domains()
which should set up the platform specific list of hmp_domains. It is
also assumed that the platform disables SD_LOAD_BALANCE for the
appropriate sched_domains.
Tasks placement takes place every time a task is to be inserted into
a runqueue based on its load history. The task placement decision is
based on load thresholds.

There are no restrictions on the number of hmp_domains, however,
multiple (>2) has not been tested and the up/down migration policy is
rather simple.

Signed-off-by: Morten Rasmussen <Morten.Rasmussen@arm.com>