sched: Add a per rq max_possible_capacity for use in power calculations

In the absence of a power driver providing real power values, the scheduler
currently defaults to using capacity of a CPU as a measure of power. This,
however, is not a good measure since the capacity of a CPU can change due
to thermal conditions and/or other hardware restrictions. These frequency
restrictions have no effect on the power efficiency of those CPUs.
Introduce max possible capacity of a CPU to track an absolute measure of
capacity which translates into a good absolute measure of power efficiency.
Max possible capacity takes the max possible frequency of CPUs into account
instead of max frequency.

Change-Id: Ia970b853e43a90eb8cc6fd990b5c47fca7e50db8
Signed-off-by: Syed Rameez Mustafa <rameezmustafa@codeaurora.org>