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: I8110811143c2854450901f76c0949f0349c4eb62
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>