powerpc/pseries: Define rtas hotplug event sections

In order to handle device hotplug in the kernel on pseries the hotplug
request will be communicated in the kernel in the form of a
rtas hotplug event. This patch adds the definition of rtas hotplug event
sections.

Signed-off-by: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>