ppc4xx: Consolidate pci_target_init() function

This patch removes the duplicted implementations of the pci_target_init()
function by introducing a weak default function for it. This weak default
has a different implementation for 440EP(x)/GR(x) PPC's. It can be
overridden by a board specific version (e.g. PMC440, korat).

Signed-off-by: Stefan Roese <sr@denx.de>
Acked-by: Matthias Fuchs <matthias.fuchs@esd.eu>