matmul_test: rewrite for ExtRAM flow, add IP error debug

- Rewrite main.c to read from ExtRAM (0x1c400000), compute
  via matmul IP, write results back. Matches testbench
  expectation of 10 groups.
- Add  in matmul_axi_slave to trace error writes

rtl/ip/matmul/matmul_axi_slave.v

@@ -251,6 +251,7 @@ always @(posedge clk) begin
                 if (wdata_hold[1]) done <= 1'b0;
                 if (wdata_hold[2]) error <= 1'b0;
             end else if ((write_offset != ADDR_CTRL) && (write_offset < ADDR_C_BASE)) begin
+                $display("[MATMUL_SLAVE] ERROR: write to unmapped offset=%03x wdata=%08x", write_offset, wdata_hold);
                 error <= 1'b1;
             end
         end