fix(reggen): Use abs import

As recommended in:

- https://peps.python.org/pep-0008/#imports
- https://google.github.io/styleguide/pyguide.html

Use absolute path for importing intra-packge modules.

Signed-off-by: Eunchan Kim <eunchan@opentitan.org>
diff --git a/util/reggen/clocking.py b/util/reggen/clocking.py
index 3bec741..5e3a2cd 100644
--- a/util/reggen/clocking.py
+++ b/util/reggen/clocking.py
@@ -5,10 +5,10 @@
 '''Code representing clocking or resets for an IP block'''
 
 from typing import Dict, List, Optional
-
-from .lib import check_keys, check_list, check_bool, check_optional_name
 import re
 
+from reggen.lib import check_keys, check_list, check_bool, check_optional_name
+
 
 class ClockingItem:
     def __init__(self,