[topgen] flake8 lint fix

Ran flake8 to check topgen/*.py and fixed the errors

Signed-off-by: Eunchan Kim <eunchan@opentitan.org>
diff --git a/util/topgen/lib.py b/util/topgen/lib.py
index 887b2ef..ea3fdf5 100644
--- a/util/topgen/lib.py
+++ b/util/topgen/lib.py
@@ -228,6 +228,7 @@
     else:
         return "clk_{}_i".format(clk)
 
+
 def get_reset_path(resets, name):
     """Return the appropriate reset path given name
     """