feat: add ML-KEM accelerator with AXI slave wrapper

Wrap mlkem_top in an AXI4 burst slave (mlkem_axi_slave) mapped to the
freed FFT slot 6 at 0x1f400000 (virtual 0xbf400000). The core runs on
sys_clk/2 (25 MHz); the wrapper handles the clock-domain crossing:
start toggle->pulse, wait-stated byte-preload handshake, busy/done
sync, and direct readback taps (ek/dk/ct/ss). CPU drives it via a
register map (CTRL/STATUS/CONFIG, d/z/msg, preload + readback windows).

Wire into soc_top on the existing axiOut_6 nets (no crossbar change),
add mlkem_intr on intrpt[3]. Add sdk mlkem_test example running a
KeyGen->Encaps->Decaps self-consistency round trip.

rtl/ip/mlkem/common/defines.vh

@@ -0,0 +1,6 @@
+`ifndef DEFINES_VH
+`define DEFINES_VH
+`define CLK_PERIOD 20.0  // 50MHz
+`define Q 3329           // ML-KEM prime modulus
+`define N 256            // polynomial degree
+`endif