[otbn] Use an absolute path for Mako template in otbn-ld

Mako complains if you use a relative path that doesn't lie in the
Python project. (Huh?)

Signed-off-by: Rupert Swarbrick <rswarbrick@lowrisc.org>
diff --git a/hw/ip/otbn/util/otbn-ld b/hw/ip/otbn/util/otbn-ld
index 943fde3..91e32ec 100755
--- a/hw/ip/otbn/util/otbn-ld
+++ b/hw/ip/otbn/util/otbn-ld
@@ -42,8 +42,8 @@
 
 
 def main() -> int:
-    ld_in = os.path.normpath(os.path.join(os.path.dirname(__file__),
-                                          '..', 'data', 'otbn.ld.tpl'))
+    ld_in = os.path.abspath(os.path.join(os.path.dirname(__file__),
+                                         '..', 'data', 'otbn.ld.tpl'))
 
     with tempfile.TemporaryDirectory(prefix='otbn-ld-') as tmpdir:
         ld_out = os.path.join(tmpdir, 'otbn.ld')