PCI: rockchip: Specify the link capability

rk3399 supports PCIe 2.x link speeds marginally at best, and on some
boards, the link won't train at 5 GT/s at all.  Rather than sacrifice 500ms
waiting for training that will never happen, let's use the helper function,
of_pci_get_max_link_speed(), to get the max link speed from DT and specify
link capability.

Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>