sunxi: use SPL header DT name for FIT board matching

Now that we can store a DT name in the SPL header, use this string (if
available) when finding the right DT blob to load for U-Boot proper.
This allows a generic U-Boot (proper) image to be combined with a bunch
of supported DTs, with just the SPL (possibly only that string) to be
different.
Eventually this string can be written after the build process by some
firmware update tool.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>