spapr_pci: Advertise access to PCIe extended config space

spapr_pci: Advertise access to PCIe extended config space

The (paravirtual) PCI host bridge on the 'pseries' machine in most
regards acts like a regular PCI bus, rather than a PCIe bus.  Despite
this, though, it does allow access to the PCIe extended config space.

We already implemented the RTAS methods to allow this access.. but
forgot to put the markers into the device tree so that guest's know it
is there.  This adds them in.

With this, a pseries guest is able to view extended config space on
(for example an e1000e device.  This should be enough to allow guests
to use at least some PCIe devices.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>