ppc/pnv: add memory regions for the ICP registers

ppc/pnv: add memory regions for the ICP registers

This provides to a PowerNV chip (POWER8) access to the Interrupt
Management area, which contains the registers of the Interrupt Control
Presenters of each thread. These are used to accept, return, forward
interrupts in the system.

This area is modeled with a per-chip container memory region holding
all the ICP registers. Each thread of a chip is then associated with
its ICP registers using a memory subregion indexed by its PIR number
in the overall region.

The device tree is populated accordingly.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>