[POWERPC] 4xx: PLB to PCI Express support

[POWERPC] 4xx: PLB to PCI Express support

This adds to the previous 2 patches the support for the 4xx PCI Express
cells as found in the 440SPe revA, revB and 405EX.

Unfortunately, due to significant differences between these, and other
interesting "features" of those pieces of HW, the code isn't as simple
as it is for PCI and PCI-X and some of the functions differ significantly
between the 3 implementations. Thus, not only this code can only support
those 3 implementations for now and will refuse to operate on any other,
but there are added ifdef's to avoid the bloat of building a fairly large
amount of code on platforms that don't need it.

Also, this code currently only supports fully initializing root complex
nodes, not endpoint. Some more code will have to be lifted from the
arch/ppc implementation to add the endpoint support, though it's mostly
differences in memory mapping, and the question on how to represent
endpoint mode PCI in the device-tree is thus open.

Many thanks to Stefan Roese for testing & fixing up the 405EX bits !

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Stefan Roese <sr@denx.de>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>