core_ctl: Add sysfs node to disable core_ctl

This core ctl enable sysfs node can be used to enable
and disable the core isolation by core ctl in runtime
for the cluster. By default enable is set to 1, core
ctl is enabled. if this gets set to 0 then isolated
cores, if any, would get available to scheduler for
load balancing and core_ctl will not do any core
isolation.

Change-Id: I856ec9358f3ceb781468e2537b4d086e4d4600f0
Signed-off-by: Sabyasachi Singh <sssingh@codeaurora.org>