arm64: add PSCI CPU_OFF-based hotplug support

This patch adds support for using PSCI CPU_OFF calls for CPU hotplug.
With this code it is possible to hot unplug CPUs with "psci" as their
boot-method, as long as there's an appropriate cpu_off function id
specified in the psci node

Change-Id: Ia1e162dbf6d77422fa41246397edd73f5d869605
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Git-commit: 831ccf79b46e02f31cfd16c66df9d5600f635155
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git


Signed-off-by: Abhimanyu Kapur <abhimany@codeaurora.org>