mpc83xx: add PCISLAVE support to 83XX_GENERIC_PCI setup code

This adds a helper function to unlock the PCI configuration bit, so that
any extra PCI setup (such as outbound windows, etc.) can be done after
using the 83XX_GENERIC_PCI code to set up the PCI bus.

Signed-off-by: Ira W. Snyder <iws@ovro.caltech.edu>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>