sunxi: board: Add a helper to get the SPL DT name

sunxi: board: Add a helper to get the SPL DT name

This moves the validity checking and typecasts all to one place away
from the string comparison logic, and it detangles the compile-time
and runtime control flow.

The new helper will also be used by U-Boot proper in a future commit.

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: protect new function with CONFIG_SPL_LOAD_FIT]
Signed-off-by: Andre Przywara <andre.przywara@arm.com>