ppc/pnv: add a PnvICPState object

ppc/pnv: add a PnvICPState object

This provides a new ICPState object for the PowerNV machine (POWER8).
Access to the Interrupt Management area is done though a memory
region. It contains the registers of the Interrupt Control Presenters
of each thread which are used to accept, return, forward interrupts in
the system.

Signed-off-by: Cédric Le Goater <[email protected]>
Reviewed-by: David Gibson <[email protected]>
Signed-off-by: David Gibson <[email protected]>