sunxi: board.c: refactor SPL header checks

So far we have two users which want to look at the SPL header. We will
get more in the future.
Refactor the existing SPL header checks into a common function, to
simplify reusing the code.
Now that this is easy, add proper version checks to the DT name parsing.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Acked-by: Jagan Teki <jagan@openedev.com>