Added missing signal modules

cores/util/mul_const/rtl/mul_const.v

@@ -0,0 +1,31 @@
+`timescale 1ns/1ps
+
+module mul_const_shiftadd #(
+    parameter integer C = 0,
+    parameter integer IN_W = 16,
+    parameter integer OUT_W = 32
+)(
+    input  wire signed [IN_W-1:0] i_x,
+    output reg  signed [OUT_W-1:0] o_y
+);
+    integer k;
+    integer abs_c;
+    reg signed [OUT_W-1:0] acc;
+    reg signed [OUT_W-1:0] x_ext;
+
+    always @* begin
+        abs_c = (C < 0) ? -C : C;
+        acc = {OUT_W{1'b0}};
+        x_ext = {{(OUT_W-IN_W){i_x[IN_W-1]}}, i_x};
+
+        for (k = 0; k < 32; k = k + 1) begin
+            if (abs_c[k])
+                acc = acc + (x_ext <<< k);
+        end
+
+        if (C < 0)
+            o_y = -acc;
+        else
+            o_y = acc;
+    end
+endmodule