PCI: fu740: Force 2.5GT/s for initial device probe

PCI: fu740: Force 2.5GT/s for initial device probe

The fu740 PCIe core does not probe any devices on the SiFive Unmatched
board without this fix (or having U-Boot explicitly start the PCIe via
either boot-script or user command). The fix is to start the link at
2.5GT/s speeds and once the link is up then change the maximum speed back
to the default.

The U-Boot driver claims to set the link-speed to 2.5GT/s to get the probe
to work (and U-Boot does print link up at 2.5GT/s) in the following code:
https://source.denx.de/u-boot/u-boot/-/blob/master/drivers/pci/pcie_dw_sifive.c?id=v2022.01#L271

Link: https://lore.kernel.org/r/20220318152430.526320-1-ben.dooks@codethink.co.uk
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>