rockchip: Cleanup of make_fit_atf.py.

This patch cleans up make_fit_atf.py in the following way:

* Fix all issues reported by pylint
* Move copyright notice from file-to-generate to script
* Fix of-by-one bugs in loadables property
* Remove commented-out (dead) code.

Besides the bugfix no intended changes.

Tested on RK3399-Q7 with TF-A v2.1 as BL31.

Signed-off-by: Christoph Muellner <christoph.muellner@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>