ppc/xive: introduce the XiveRouter model

ppc/xive: introduce the XiveRouter model

The XiveRouter models the second sub-engine of the XIVE architecture :
the Interrupt Virtualization Routing Engine (IVRE).

The IVRE handles event notifications of the IVSE and performs the
interrupt routing process. For this purpose, it uses a set of tables
stored in system memory, the first of which being the Event Assignment
Structure (EAS) table.

The EAT associates an interrupt source number with an Event Notification
Descriptor (END) which will be used in a second phase of the routing
process to identify a Notification Virtual Target.

The XiveRouter is an abstract class which needs to be inherited from
to define a storage for the EAT, and other upcoming tables.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
[dwg: Folded in parts of a later fix by Cédric fixing field access]
[dwg: Fix style nits]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>