sched: Add min_max_freq and rq->max_possible_freq

rq->max_possible_freq represents the maximum frequency a cpu is
capable of attaining, while rq->max_freq represents the maximum
frequency a cpu can attain at a given instant. rq->max_freq includes
constraints imposed by user or thermal driver.
rq->max_freq <= rq->max_possible_freq.

max_possible_freq is derived as max(rq->max_possible_freq) and
represents the "best" cpu that can attain best possible frequency.

min_max_freq is derived as min(rq->max_possible_freq). For homogeneous
systems, max_possible_freq and min_max_freq will be same, while they
could be different on heterogeneous systems.

Change-Id: Iec485fde35cfd33f55ebf2c2dce4864faa2083c5
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>