sunxi: Extend SPL header versioning

On Allwinner SoCs we use some free bytes at the beginning of the SPL image
to store various information. We have a version byte to allow updates,
but changing this always requires all tools to be updated as well.

Introduce the concept of semantic versioning [1] to the SPL header:
The major part of the version number only changes on incompatible
updates, a minor number bump indicates backward compatibility.
This patch just documents the major/minor split, adds some comments
to the header file and uses the versioning information for the existing
users.

[1] https://semver.org



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>