sunxi: configs: streamline include/configs/sun*.h wrappers

For mostly historic reasons we had configuration headers for each
Allwinner CPU "family". These days they are mostly just including one
common header, with the rest being somewhat empty.
There were attempts to remove them, and to just use the one common header
to begin with, but this has implications to the build system, which me
might not be ready for, yet.

To document this behaviour, and to avoid something sneaking in over
time, make those files all the same (minus the CPU family name and
the copyrights), and add a comment explaining that.
This makes it easier to just remove those files later on, when needed
and possible.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Tom Rini <trini@konsulko.com>