timingconstraints_lpgbt fix for phase2/master

Hi Frans,

the timing_constraints_lpgbt.xdc was heavily revised. More info in https://its.cern.ch/jira/browse/FLX-1334. I propose to fix the https://gitlab.cern.ch/atlas-tdaq-felix/firmware/-/blob/phase2/master/constraints/timing_constraints.xdc#L44-148 according to what was done in https://gitlab.cern.ch/atlas-tdaq-felix/firmware/-/blob/phase2/master_timingconstraintsFix/constraints/timing_constraints_lpgbt.xdc#L24-120 (use pins which are inputs to the BUFG_GTs rather than outputs). Without the fix some of the set_max_delay and other commands would not execute correctly.

Marco