powerpc/pseries: Add support for hotplug interrupt source

Add handler for new hotplug interrupt. For memory and CPU hotplug events,
we will add the hotplug errorlog to the hotplug workqueue. Since PCI
hotplug is not currently supported in the kernel, PCI hotplug events are
written to the rtas_log_bug and are handled by rtas_errd.

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>