powerpc/powernv: Skip check on PE if necessary

While the device driver or PCI core tries to enable PCI device, the
platform dependent callback "ppc_md.pcibios_enable_device_hook" will
be called to check if there has one associated PE for the PCI device.
If we don't have the associated PE for the PCI device, it's not allowed
to enable the PCI device. Unfortunately, there might have some cases
we have to enable the PCI device (e.g. P2P bridge), but the PEs have
not been created yet.

The patch handles the unfortunate cases. Each PHB (struct pnv_phb)
has one field "initialized" to trace if the PEs have been created
and configured or not. When the PEs are not available, we won't check
the associated PE for the PCI device to be enabled.

Signed-off-by: Gavin Shan <shangw@linux.vnet.ibm.com>
Reviewed-by: Ram Pai <linuxram@us.ibm.com>
Reviewed-by: Richard Yang <weiyang@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>