scripts/liberty: Make the `clk_width` attribute a string.

Fixes #69.

Signed-off-by: Tim 'mithro' Ansell <me@mith.ro>
diff --git a/scripts/python-skywater-pdk/skywater_pdk/liberty.py b/scripts/python-skywater-pdk/skywater_pdk/liberty.py
index c2c8de7..e6904c2 100755
--- a/scripts/python-skywater-pdk/skywater_pdk/liberty.py
+++ b/scripts/python-skywater-pdk/skywater_pdk/liberty.py
@@ -810,7 +810,7 @@
 
             elif "clk_width" == ktype:
                 for l in sorted(v):
-                    o.append("%s%s : %s;" % (INDENT*len(indent_n), k, l))
+                    o.append('%s%s : "%s";' % (INDENT*len(indent_n), k, l))
 
             else:
                 raise ValueError("Unknown %s: %r\n%s" % (k, v, indent_n))