sched: Make RAVG_HIST_SIZE tunable

Make RAVG_HIST_SIZE available from /proc/sys/kernel/sched_ravg_hist_size
to allow tuning of the size of the history that is used in computation
of task demand.

CRs-fixed: 706138
Change-Id: Id54c1e4b6e974a62d787070a0af1b4e8ce3b4be6
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>