rockchip: add a common script for generate fit its

Rockchip release bl31.elf file for armv8 SoCs like rk3399, rk3328,
the elf have more than one section, we need to decode it first and
packed them into u-boot.itb with its file. This script is to generate
the its script.
Need default bl31.elf in root directory of U-Boot source and dtb
as parameter.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>