arm64: kpti: force off kpti

Since most of the CPUs aren't prone to meltdown,
force off the kpti feature. Enable on required
targets.

Change-Id: If78c70bcaeac2b157a901e5036190aaf9e5aabf6
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>