pci-host: Allow extended config space access for PowerNV PHB4 model

pci-host: Allow extended config space access for PowerNV PHB4 model

The PCIe extended configuration space on the device is not currently
accessible to the host. if by default,  it is still inaccessible for
conventional for PCIe buses, add the current flag
PCI_BUS_EXTENDED_CONFIG_SPACE on the root bus permits PCI-E extended
config space access.

Signed-off-by: Christophe Lombard <clombard@linux.vnet.ibm.com>
Reviewed-by: Frederic Barrat <fbarrat@linux.ibm.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20211109145053.43524-1-clombard@linux.vnet.ibm.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>