Add basic output on success

Whilst it is true that "silence is golden", no output at all could be
disconcerting and it makes searching in a CI log more difficult.

Add a simple status message that says "Checked N files, style ok".

Signed-off-by: David Horstmann <david.horstmann@arm.com>

scripts/code_style.py

@@ -199,6 +199,8 @@ def main() -> int:
     else:
         # Check mode
         if check_style_is_correct(src_files):
+            print("Checked {} files, style ok.".format(len(src_files)),
+                  file=STDOUT_UTF8)
             return 0
         else:
             return 1