rockchip: make_fit_atf: explicitly use python3

On a distribution with no python2 installed and no
python->python3 symlink the script will fail to execute.
Specify python3 explicitly as it's already a requirement
to build u-boot.
Signed-off-by: NJack Mitchell <ml@embed.me.uk>
Reviewed-by: NKever Yang <kever.yang@rock-chips.com>

arch/arm/mach-rockchip/make_fit_atf.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 """
 # SPDX-License-Identifier: GPL-2.0+
 #