sched: walt: Add CFS_BANDWIDTH support

Update the walt stats when a fair class group is throttled and
unthrottled. Avoid migrating a task in the tick path if the
task is about to be throttled.

When a cfs_rq is throttled, remove its contribution from the
cumulative window demand so that the same can be added
unconditionally when the cfs_rq is unthrottled again. Otherwise
we have to keep track of the window when the cfs_rq is throttled
and unthrottled.

Change-Id: Id6299d27e26139cb17114eda8ad82c347dcb713b
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>