Add comment explaining generate_psa_code.py

Explain that the output filename is derived from the -d
argument, so that it's obvious why the CMakefile code
does what it does.

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

tests/CMakeLists.txt

@@ -92,6 +92,8 @@ function(add_test_suite suite_name)
 
     add_custom_command(
         OUTPUT
+            # The output filename of generate_test_code.py is derived from the -d
+            # input argument.
             test_suite_${data_name}.c
         COMMAND
             ${MBEDTLS_PYTHON_EXECUTABLE}