sunxi: board: Set fdtfile to match the DT chosen by SPL

sunxi: board: Set fdtfile to match the DT chosen by SPL

Previously, fdtfile was always the value in CONFIG_DEFAULT_DEVICE_TREE.
This meant that, regardless of the DT chosen by SPL (either by changing
the header in the image or by the selection code at runtime), Linux
always used the default DT.

By using the name from the SPL header (which, because of the previous
commit, always matches the DT used by U-Boot proper), Linux also sees
the same board as U-Boot/SPL, even if the boot script later loads a DT
from disk.

Acked-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
[Andre: remove no longer needed CONFIG_SPL_LOAD_FIT guards]
Signed-off-by: Andre Przywara <andre.przywara@arm.com>