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/ip/bram/init_hex.mem

@@ -1,34 +1,34 @@
 a9
-ff
+7b
 8d
-fa
+f0
+ef
+a9
+00
+8d
+f1
+ef
+a9
+c8
+8d
+f2
 ef
 a9
 01
 8d
-fb
+f3
+ef
+ad
+f5
 ef
-a9
-00
 8d
-f9
+ff
 ef
-a9
-10
-8d
-f8
-ef
-58
 cb
 80
-fd
-ad
-f8
-ef
-ee
-ff
-ef
-40
+e3
+00
+00
 00
 00
 00
@@ -252,5 +252,5 @@ ef
 ff
 00
 ff
-18
+00
 ff