sched: fix init NOHZ_IDLE flag

On my smp platform which is made of 5 cores in 2 clusters, I have the
nr_busy_cpu field of sched_group_power struct that is not null when the
platform is fully idle. The root cause seems to be:
During the boot sequence, some CPUs reach the idle loop and set their
NOHZ_IDLE flag while waiting for others CPUs to boot. But the nr_busy_cpus
field is initialized later with the assumption that all CPUs are in the busy
state whereas some CPUs have already set their NOHZ_IDLE flag.
We set the NOHZ_IDLE flag when nr_busy_cpus is initialized to 0 in order to
have a coherent configuration.
If set_cpu_sd_state_idle is called during the buld of the new sched_domain it
will not crrupt the initial state
set_cpu_sd_state_busy is modified and clear the NOHZ_IDLE only if a non NULL
sched_domain is attached to the CPU.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: franciscofranco <franciscofranco.1990@gmail.com>