ARM: rockchip: Add cpu hotplug support for RK3XXX SoCs

Adds ability to shutdown all CPUs except the first one
(since it might be special for a lot of platforms).
It is now possible to use kexec which requires such a feature.

Signed-off-by: Romain Perier <romain.perier@gmail.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>