Documentation: sunxi: Add A80 datasheet link

We now have initial support for the A80, as well a the datasheet.
Update the documents to reflect this.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Tested-by: Andreas Färber <afaerber@suse.de>