Adds PCI support for MPC5121

Tested with drivers/net/rtl8139.c

Support is conditional since PCI on old silicon does not work.

ads5121_PCI_config turns on PCI

In this version, condition compilation of PCI code has been moved
from ifdef in board/ads5121/pci.c to board/ads5121/Makefile as
suggested by Jean-Christophe PLAGNIOL-VILLARD

Signed-off-by: John Rigby <jrigby@freescale.com>