wr_board_pkg: declare eb_ethernet_slave as a component so that compiler doesn't complain when EB repo is not included

based on g_pcs_16bit

Provide the rate of the WR Reference Clock based on the information
about the width of the PCS word. It is assumed to be related:
* 16bit word with 62.5MHz clock
*  8bit word with  125MHz clock

WR Streamers need to be used with WR Reference clock of 62.5MHz,
adding generic to specify what ref_clk is used (125MHz by default,
or 62.5MHz)

The generic g_ref_clk_rate was dummy, i.e. never used. The module
pulse_stamper is used with input reference clock (and tm_cycles_i)
of 125MHz and 62.5MHz clock, in the wr_streamers. Added possibility
to define what clock is used (default 125MHz or 62.5MHz). In any
case, the output timestamp is of cycle period of 8ns.