sched: add sched_get_cpu_last_busy_time() API

sched_get_cpu_last_busy_time() returns the last time stamp when
a given CPU is busy with more than 2 runnable tasks or has load
greater than 50% of it's max capacity. The LPM driver can make
use of this API and create a policy to prevent a recently loaded
CPU entering deep sleep state.

This API is implemented only for the higher capacity CPUs in
the system. It returns 0 for other CPUs.

Change-Id: I97ef47970a71647f4f55f21165d0cc1351770a53
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>