sched: add stub for sched_set_io_is_busy

The sched_set_io_is_busy function is used by an upcoming
cpufreq patch but the functionality is not complete yet.

Until the actual implementation is complete, idle cycles
will not be accounted as busy time when the CPU becomes
idle as a result of an io_schedule() call.

Change-Id: I2dacbffa491a2f768a4389f88e1fd2d148eaae7c
Signed-off-by: Steve Muckle <smuckle@codeaurora.org>