powerpc/p4080ds: fix PCI-e x8 link training down failure

powerpc/p4080ds: fix PCI-e x8 link training down failure

Due to SerDes configuration error, if we set the PCI-e controller link width
as x8 in RCW and add a narrower width(such as x4, x2 or x1) PCI-e device to
PCI-e slot, it fails to train down to the PCI-e device's link width. According
to p4080ds errata PCIe-A003, we reset the PCI-e controller link width to x4 in
u-boot. Then it can train down to x2 or x1 width to make the PCI-e link between
RC and EP.

Signed-off-by: Yuanquan Chen <B41889@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>