Added LpGBT support for VC709

Hi Frans,

Merge request associated to FLX-1533 is now ready.

Please notice that large negative WNS/TNS are observed for the GBT (same findings in phase2/master). I suspect the set_false_paths between clk40 and TX/RXOUTCLK went missing. Moreover, as a minor comment, get_ports opto_los[*] (https://gitlab.cern.ch/atlas-tdaq-felix/firmware/-/blob/phase2/master/constraints/felix_gbt_sfp_VC709.xdc#L27-37) are only generating critical warnings: are they really needed? Vivado must be optimizing those out, but I'm not sure why since they are used in FELIX_gbt_wrapper_V7

Thanks Best, Marco