sunxi: 64-bit SoCs: introduce FIT generator script

Now that the Makefile can call a generator script to build a more
advanced FIT image, let's use this feature to address the needs of
Allwinner boards with 64-bit SoCs (A64 and H5).
The (DTB stripped) U-Boot binary and the ATF are static, but we allow
an arbitrary number of supported device trees to be passed.
The script enters both a DT entry in the /images node and the respective
subnode in /configurations to support all listed DTBs.

The location of the bl31.bin image from the ARM Trusted Firmware build
can either by specified via the BL31 environment variable. If this is not
set, the script looks for bl31.bin in U-Boot's build directory (which
could be a symlink as well).

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>