cpu hotplug: add multi instance support

[Sebastian Andrzej Siewior]

This series adds the multi instance support for a single hotplug state. We have
currently three drivers in tree which have their own list in order to handle
their individual instances.
There will be probably more of these drivers than less so we try to shift the
bits and pieces into the core code. The three ARM drivers are converted as part
of this series but only compile tested. The virtio-net was used to test patch
one and two.

Sebastian