sched: Introduce the concept CPU clusters in the scheduler

A cluster is set of CPUs sharing some power controls and an L2 cache.
This patch buids a list of clusters at bootup which are sorted by
their max_power_cost. Many cluster-shared attributes like cur_freq,
max_freq etc are needlessly maintained in per-cpu 'struct rq' currently.
Consolidate them in a cluster structure.

Change-Id: I0567672ad5fb67d211d9336181ceb53b9f6023af
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>
Signed-off-by: Syed Rameez Mustafa <rameezmustafa@codeaurora.org>