PCI: Move Open Firmware devspec attribute to PCI common code

Move the devspec OF attribute to PCI common code's set of device attributes
since it's not architecture dependent.  As a side effect microblaze and
powerpc no longer need to use pcibios_add_platform_entries().

[bhelgaas: fold in #include for compile error]
Link: https://lkml.kernel.org/r/alpine.LFD.2.11.1404141101500.1529@denkbrett


Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>