sunxi: binman: Respect the default FIT configuration

binman can fill in the default FIT configuration index as selected by
the "default-dt" argument, which is set to CONFIG_DEFAULT_DEVICE_TREE.
Let's respect the user's configuration by taking advantage of this
feature, instead of always defaulting to the first device tree in
CONFIG_OF_LIST.

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