sunxi: board: Save the chosen DT name in the SPL header

sunxi: board: Save the chosen DT name in the SPL header

This overwrites the name loaded from the SPL image. It will be different
if there was previously no name provided, or if a more accurate name was
determined by the board variant selection logic. This means that the DT
name in the SPL header now always matches the DT appended to U-Boot.

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: move function under CONFIG_SPL_LOAD_FIT guard]
Signed-off-by: Andre Przywara <andre.przywara@arm.com>