pci/layerscape: fix link and class issues to support ls2085a

1. LS2085a provides PCIE_LUT_DBG register rather than PCIE_LDBG
   to show the link status, so the patch fixes it.
2. Increase the delay time to make sure that link training
   has finished.
3. Return invalid value when accessing multi-function device
4. For LS2085a DBI_RO_WR_EN bit is cleared as default, so we
   must set this bit before change DBI register value.

Signed-off-by: Roy Zang <tie-fei.zang@freescale.com>
Signed-off-by: Minghuan Lian <Minghuan.Lian@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>