Add multiplier

Add 16x16 multiplier.

Pretty simple. Address 0-1 is multipled by address 2-3 and the result is
in address 4-7, all little endian of course.

hw/efinix_fpga/addr_decode.sv

@@ -5,12 +5,14 @@ module addr_decode
     output o_rom_cs,
     output o_leds_cs,
     output o_timer_cs,
+    output o_multiplier_cs,
     output o_sdram_cs
 );
 
 assign o_rom_cs = i_addr >= 16'hf000 && i_addr <= 16'hffff;
-assign o_leds_cs = i_addr == 16'hefff;
 assign o_timer_cs = i_addr >= 16'heff8 && i_addr <= 16'heffb;
+assign o_multiplier_cs = i_addr >= 16'heff0 && i_addr <= 16'heff7;
+assign o_leds_cs = i_addr == 16'hefff;
 assign o_sdram_cs = i_addr < 16'h8000;
 
 endmodule