ppc4xx: Make PCIE support selectable

On some platforms PCIE support is not required, but would be included
because the cpu supports it. To reduce fooprint it is now configurable
via CONFIG_PCI_DISABLE_PCIE.

Signed-off-by: Dirk Eibach <eibach@gdsys.de>
Signed-off-by: Stefan Roese <sr@denx.de>