powerpc/powernv: Create bus sensitive PEs

Basically, there're 2 types of PCI bus sensitive PEs: (A) The PE
includes single PCI bus. (B) The PE includes the PCI bus and all
the subordinate PCI buses. At present, we'd like to put PCI bus
originated by PCI-e link to form PE that contains single PCI bus,
and the PCIe-to-PCI bridge will form the 2nd type of PE. We don't
figure out to detect PLX bridge yet. Once we can detect PLX bridge
some day, we have to put PCI buses originated from the downstream
port of PLX bridge to the 2nd type of PE.

The patch changes the original implementation for a little bit
to support 2 types of PCI bus sensitive PEs described as above.
Also, the function used to retrieve the corresponding PE according
to the given PCI device has been changed based on that because each
PCI device should trace the directly associated PE.

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>