⚠️ [rtl] rework SLINK module

[stnolting]

This is just another rework PR that aims to simplify the processor's SLINK module. Furthermore, support for the AXI Stream-compatible T_LAST signals is added.

🚀 New Features

Add stream link "last" signals to indicate end of packet. New top signals:

slink_tx_lst_o : out std_ulogic_vector(7 downto 0); -- last data of packet

slink_rx_lst_i : in  std_ulogic_vector(7 downto 0) := (others => 'L'); -- last data of packet

Example transmission using "last" signal:

⚠️ Major Changes

• removed all "FIFO half full" status flags and interrupt triggers
• simplified interrupt configuration
• removed SLINK IRQ register
• reorganized CTRL and STATUS registers (bits, flag position, memory-mapped addresses)