ARM: highbank: Add cpu hotplug support

This adds cpu hotplug for highbank. On highbank, a core is always reset and
boots up the same path as a cold boot.

Signed-off-by: Martin Bogomolni <martin@calxeda.com>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Reviewed-by: Jamie Iles <jamie@jamieiles.com>
Reviewed-by: Shawn Guo <shawn.guo@linaro.org>