ppc/xive: Handle END triggers between chips with MMIOs

ppc/xive: Handle END triggers between chips with MMIOs

The notify page of the interrupt controller can either be used to
receive trigger events from the HW controllers (PHB, PSI) or to
reroute interrupts between Interrupt Controllers. In which case, the
VSD table is used to determine the address of the notify page of the
remote IC and the store data is forwarded.

Today, our model grabs the remote VSD (EAS, END, NVT) address using
pnv_xive_get_remote() helper. Be more precise and implement remote END
triggers using a store on the remote IC notify page.

We still have a shortcut in the model for the NVT accesses which we
will address later.

Reviewed-by: Frederic Barrat <fbarrat@linux.ibm.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>