[SLINK] add AXI-stream-compatible "tlast" signals

[stnolting]

Add native hardware support for the AXI-stream-compatible tlast signals that are used to identify the last data word of a stream.

Add new top signals:

  slink_rx_lst_i : in  std_ulogic := 'L'; -- last element of stream
  slink_tx_lst_o : out std_ulogic; -- last element of stream

⚠️ Reworked SLINK low-level driver / HAL. The SLINK module now uses four memory-mapped interface registers for control and data transfers. See the updated documentation.

ℹ️ The updated HAL is fully backwards-compatible.

🚀 Discussed in #795; triggered by @Giorgio122.