sched: Extend boost benefit for small and low-prio tasks

Allow small and low-prio tasks to benefit from boost, which is
expected to last for a short duration. Any task that wishes to run
during that short period is allowed boost benefit.

Change-Id: I02979a0c5feeba0f1256b7ee3d73f6b283fcfafa
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>