gen: add common with reverse_bits/reverse_bytes functions

litex/gen/__init__.py

@@ -1 +1,2 @@
 from litex.gen.sim import *
+from litex.gen.common import *

litex/gen/common.py

@@ -0,0 +1,11 @@
+from migen import *
+
+
+def reverse_bits(s):
+    return s[::-1]
+
+
+def reverse_bytes(s):
+    n = (len(s) + 7)//8
+    return Cat(*[s[i*8:min((i + 1)*8, len(s))]
+        for i in reversed(range(n))])