sched: colocate related threads

Provide userspace interface for tasks to be grouped together as
"related" threads. For example, all threads involved in updating
display buffer could be tagged as related.

Scheduler will attempt to provide special treatment for group of
related threads such as:

1) Colocation of related threads in same "preferred" cluster
2) Aggregation of demand towards determination of cluster frequency

This patch extends scheduler to provide best-effort colocation support
for a group of related threads.

Change-Id: Ic2cd769faf5da4d03a8f3cb0ada6224d0101a5f5
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>
Signed-off-by: Syed Rameez Mustafa <rameezmustafa@codeaurora.org>