hw/ppc/ppc4xx_pci: Fix ppc4xx_pci_map_irq() for recent Linux kernels

hw/ppc/ppc4xx_pci: Fix ppc4xx_pci_map_irq() for recent Linux kernels

Recent Linux kernels are accessing the PCI device in slot 0 that
represents the PCI host bridge. This causes ppc4xx_pci_map_irq()
to return -1 which causes an assert() later:

 hw/pci/pci.c:262: pci_bus_change_irq_level: Assertion `irq_num >= 0' failed.

Thus we should allocate an IRQ line for the device in slot 0, too.
To avoid changes to the outside of ppc4xx_pci.c, we map it to
the internal IRQ number 4 which will then happily be ignored since
ppc440_bamboo.c does not wire it up.

With these changes it is now possible again to use recent Linux
kernels for the bamboo board.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20211019091817.469003-1-thuth@redhat.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Tested-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>