Support more than Gen2 speed link mode, since i.MX8MP PCIe supports up to
Gen3 link speed.

Link: https://lore.kernel.org/r/1658287576-26908-1-git-send-email-hongxing.zhu@nxp.comSigned-off-by: NRichard Zhu <hongxing.zhu@nxp.com>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>