[wr-streamers] bugfix and added reset of stats after (re-)start and synch

1) bugfix:
   - tm_time_valid_i signal from ref_clk domain was used in sys_clk
     to gate statistics in xrx_streamers_stats.vhd - likely this gating
     was not needed because statistics are done when rcvd_latency_valid_i
     is true and this should be true only when time is valid, anyway
   - added gc_sync_ffs to synchronize tm_time_valid_i to sys_clk
     (added in xrtx_streamers_stats.vhd

2) new feature
   - added initial reset of statistics when the WR node is first
     synchronized after powerup/reset.
   - this is useful so that we have a valid reset timestamp without
     a need for a management tool to reset them. So far, when a device
     was started, the reset timestamp was zero, so without additional
     action of reseting stats, it was not possible to know when the
     stats were started