sched: Track number of big and small tasks on a cpu

This patch adds 'nr_big_tasks' and 'nr_small_tasks' per-cpu counters
that tracks number of big and small tasks on a cpu respectively. This
will be used in load balance decisions introduced in a subsequent
patch.

Change-Id: Ia174904140f81dd6d1946286889a50be3f16ea83
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>