powerpc/4xx: Allow 4xx PCI bridge to be disabled via device tree

This patch allows the 4xx (conventional) PCI bridge to be disabled
via the device tree. This is needed for 4xx PCI adapter hardware.

Use the PCI node's status property to disable the PCI bridge.

Signed-off-by: Matthias Fuchs <matthias.fuchs@esd-electronics.com>
Acked-by: Stefan Roese <sr@denx.de>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>