PCI: imx6: Reduce resume time by only starting link if it was up before suspend

i.MX PCIe doesn't support hotplug. During resume, only start PCIe link
training when the link was up before system suspend to avoid the long
latency in the link training period.

Link: https://lore.kernel.org/r/1657783869-19194-14-git-send-email-hongxing.zhu@nxp.com


Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>