ppc/pnv: Fix PCI_EXPRESS dependency

When PHB4 bridge has been added, the dependencies to PCIE_PORT has been
added to XIVE_SPAPR and indirectly to PSERIES.
The build of the PowerNV machine is fine while we also build the PSERIES
machine.
If we disable the PSERIES machine, the PowerNV build fails because the
PCI Express files are not built:

/usr/bin/ld: hw/ppc/pnv.o: in function `pnv_chip_power8_pic_print_info':
.../hw/ppc/pnv.c:623: undefined reference to `pnv_phb3_msi_pic_print_info'
/usr/bin/ld: hw/ppc/pnv.o: in function `pnv_chip_power9_pic_print_info':
.../hw/ppc/pnv.c:639: undefined reference to `pnv_phb4_pic_print_info'
/usr/bin/ld: ../hw/usb/hcd-ehci-pci.o: in function `usb_ehci_pci_write_config':
.../hw/usb/hcd-ehci-pci.c:129: undefined reference to `pci_default_write_config'
/usr/bin/ld: ../hw/usb/hcd-ehci-pci.o: in function `usb_ehci_pci_realize':
.../hw/usb/hcd-ehci-pci.c:68: undefined reference to `pci_allocate_irq'
/usr/bin/ld: .../hw/usb/hcd-ehci-pci.c:72: undefined reference to `pci_register_bar'
/usr/bin/ld: ../hw/usb/hcd-ehci-pci.o:(.data.rel+0x50): undefined reference to `vmstate_pci_device'

This patch fixes the problem by adding needed dependencies to POWERNV.

Fixes: 4f9924c4d4cf ("ppc/pnv: Add models for POWER9 PHB4 PCIe Host bridge")
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Message-Id: <20200205232016.588202-3-lvivier@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>