powerpc/pseries: Add pseries hotplug workqueue

In support of PAPR changes to add a new hotplug interrupt, introduce a
hotplug workqueue to avoid processing hotplug events in interrupt context.
We will also take advantage of the queue on PowerVM to ensure hotplug
events initiated from different sources (HMC and PRRN events) are handled
and serialized properly.

Signed-off-by: John Allen <jallen@linux.vnet.ibm.com>
Reviewed-by: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>