alveo_u250: Add PCIe x4 support

[daveshah1]

Based on the implementation in xcu1525.

Note that enjoy-digital/litepcie#39 needs to be fixed before this builds out-of-the-box.

scratch_test seems to work correctly:

$ ./litepcie_util scratch_test
Write 0x12345678 to scratch register:
Read: 0x12345678
Write 0xdeadbeef to scratch register:
Read: 0xdeadbeef

I haven't tested any further than that. I also found I needed to reboot for the device to appear correctly, but my setup is somewhat unusual as it involves Thunderbolt on a X399 motherboard without the proper BIOS support.

[daveshah1]

I also found I needed to reboot for the device to appear correctly, but my setup is somewhat unusual as it involves Thunderbolt on a X399 motherboard without the proper BIOS support.

The solution to this is almost enjoy-digital/litepcie#36 (comment), but in my case I have to direct the rescan just to the Alpine Ridge device and not the whole PCI subsystem; or my machine totally locks up.

[enjoy-digital]

Thanks, merged. The naming issue with be fixed soon and more testing will be done in the next days with LitePCIe/Ultrascale+ devices.