ppc4xx: Add pci_pre_init() for 405 boards

This patch adds support for calling a plattform dependant
pci_pre_init() function for 405 boards. This can be used to
move the current pci_405gp_fixup_irq() function into the
board code.

This patch also makes the CFG_PCI_PRE_INIT define obsolete.
A default function with 'weak' attribute is used when
a board specific pci_pre_init() is not implemented.

Signed-off-by: Matthias Fuchs <matthias.fuchs@esd-electronics.com>