sched: walt: refactor the walt stats update code

Each scheduling class has its own wrappers to update/fixup
the walt stats. Move the update/fixup logic to a common
place (walt.c and walt.h).

Change-Id: I405874ad7bb537d87740ad9f0d1f6e72fc26fd8b
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>