add no dsp hint for FFT

rtl/ip/fft/SdfUnit.v

@@ -1,6 +1,7 @@
 //----------------------------------------------------------------------
 //  SdfUnit: Radix-2^2 Single-Path Delay Feedback Unit for N-Point FFT
 //----------------------------------------------------------------------
+(* use_dsp = "no" *)
 module SdfUnit #(
     parameter   N = 64,     //  Number of FFT Point
     parameter   M = 64,     //  Twiddle Resolution
@@ -85,7 +86,7 @@ reg             bf2_do_en;  //  2nd Butterfly Output Data Enable
 //  Multiplication
 wire[1:0]       tw_sel;     //  Twiddle Select (2n/n/3n)
 wire[LOG_N-3:0] tw_num;     //  Twiddle Number (n)
-wire[LOG_N-1:0] tw_addr;    //  Twiddle Table Address
+(* use_dsp = "no" *) wire[LOG_N-1:0] tw_addr;    //  Twiddle Table Address
 wire[WIDTH-1:0] tw_re;      //  Twiddle Factor (Real)
 wire[WIDTH-1:0] tw_im;      //  Twiddle Factor (Imag)
 reg             mu_en;      //  Multiplication Enable