sched: defining relaxed version of idle_cpu

Defining relaxed version of idle_cpu, which uses
macro cpu_relaxed_read_long, that will be used to
enhance power efficiency.

bug 1440421

Signed-off-by: Sumit Singh <sumsingh@nvidia.com>
Reviewed-by: Sri Krishna Chowdary <schowdary@nvidia.com>
Reviewed-by: Alexander Van Brunt <avanbrunt@nvidia.com>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>