sched: Extend update_task_ravg() to accept wallclock as argument

This will make it easier to account interrupt time on a cpu,
introduced in a subsequent patch.

Change-Id: I0e1fb5255c280ca374fd255e7fc19d5de9f8b045
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>