Investigate possibility to wire the PCIe to the PL GTH

Currently we use the PS MGT for the PCIe link to the PXIe backplane, but the PL could provide much faster speeds and support newer PCIe generations.

At the same time, see if it makes sense to provide some of the PL GTH (and/or the PS MGT if we replace it) using on-board connectors, like for example Samtec FireFly.

It's important to evaluate the changes not only at the level of hardware and FPGA, but also Linux drivers. If our current DMA engine will not work with the PL, then that's something to really take into account.