AXI4-Lite crossbar error reporting

[pvantrap]

A follow-up of #7 from @lorenzschmid , I activated bus-error (I also have busgroup) for a crossbar and the containing submaps and while it works for the submaps, the crossbar always returns OKAY on bresp and rresp:

   -- AW, W and B channels
  axi4l_o.awready <= not axi_awset;
  axi4l_o.wready <= not axi_wset;
  axi4l_o.bvalid <= axi_wdone;
  process (aclk) begin
  ...
  end process;
  axi4l_o.bresp <= "00";

  -- AR and R channels
  axi4l_o.arready <= not axi_arset;
  axi4l_o.rvalid <= axi_rdone;
  process (aclk) begin
 ...
  end process;
  axi4l_o.rresp <= "00";

I think there's two additions to be made:

• pass the submap's bresp and rresp when used
• reply SLVERR in case of addressing is not covered by any submap's range

Making this issue for completeness and documentation, I don't have time to investigate further or implement with a testbench right away.

For info @tgingold-cern