ppc4xx: Make 440SPe PCIe code more generic to use on different 4xx PPCs (2)

This patch is the first patch of a series to make the 440SPe PCIe code
usable on different 4xx PPC platforms. In preperation for the new 405EX
which is also equipped with PCIe interfaces.

(2) This patch renames the functions from 440spe_ to 4xx_ with a
    little additional cleanup

Signed-off-by: Stefan Roese <sr@denx.de>