core_ctl: Implement not_preferred hint strictly

Don't isolate CPUs other than not_preferred CPUs unless limits
(max_cpus) condition is not met. All CPUs are eligible for
isolation when there is no preference. This policy allows
not_preferred tunable to be used to specify which CPUs need to
be kept active.

Change-Id: I6151067e3e55e5751b16f02212d9332fc3b7809b
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>