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/gen_sec_cm_testplan.py b/util/reggen/gen_sec_cm_testplan.py
index c91bb14..e6e9493 100644
--- a/util/reggen/gen_sec_cm_testplan.py
+++ b/util/reggen/gen_sec_cm_testplan.py
@@ -11,7 +11,7 @@
 from mako.lookup import TemplateLookup  # type: ignore
 from pkg_resources import resource_filename
 
-from .ip_block import IpBlock
+from reggen.ip_block import IpBlock
 
 
 def gen_sec_cm_testplan(block: IpBlock, outdir: str) -> int: