Add usage info of generate_psa_constants script

Signed-off-by: Hugues de Valon <hugues.devalon@arm.com>

scripts/generate_psa_constants.py

@@ -1,9 +1,14 @@
 #!/usr/bin/env python3
 
-"""Generate programs/psa/psa_constant_names_generated.c
+"""Generate psa_constant_names_generated.c
 which is included by programs/psa/psa_constant_names.c.
 The code generated by this module is only meant to be used in the context
 of that program.
+
+An argument passed to this script will modify the output directory where the
+file is written:
+* by default (no arguments passed): writes to programs/psa/
+* OUTPUT_FILE_DIR passed: writes to OUTPUT_FILE_DIR/
 """
 
 import os