powerpc/powernv: Add XICS emulation APIs

OPAL provides an emulated XICS interrupt controller to
use as a fallback on newer processors that don't have a
XICS. It's meant as a way to provide backward compatibility
with future processors. Add the corresponding interfaces.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Stewart Smith <stewart@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>