target/ppc: Add POWER9 external interrupt model

target/ppc: Add POWER9 external interrupt model

Adds support for the Hypervisor directed interrupts in addition to the
OS ones.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
[clg: - modified the icp_realize() and xive_tctx_realize() to take
        into account explicitely the POWER9 interrupt model
      - introduced a specific power9_set_irq for POWER9 ]
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20190215161648.9600-10-clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>