sched: maintain group busy time counters in runqueue

There is no advantage of tracking busy time counters per related
thread group. We need busy time across all groups for either a CPU
or a frequency domain. Hence maintain group busy time counters in
the runqueue itself. When CPU window is rolled over, the group busy
counters are also rolled over. This eliminates the overhead of
individual group's window_start maintenance.

As we are preallocating related thread group now, this patch saves
40 * nr_cpu_ids * (nr_grp - 1) bytes memory.

Change-Id: Ieaaccea483b377f54ea1761e6939ee23a78a5e9c
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>