sunxi: binman: Do not hardcode U-Boot load address

The FIT description has access to the configuration variables. Use the
appropriate variable instead of hardcoding the address.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>