Fix issues with lit-exec-fvp.py (#625)

This patch gives execution permissions to this Python script and fixes
one wrong variable name.

Since this script is used as an executable in our lit tests, it ought to
have execution rights.

Author: vhscampos

arm-runtimes/test-support/lit-exec-fvp.py

@@ -26,7 +26,7 @@ def main():
         help="Directory containing FVP config files",
         required=True,
     )
-    main_arg_group.add_argument(
+    parser.add_argument(
         "--fvp-model",
         help="model name for FVP",
         required=True,