Documentation: sunxi: Update Allwinner SoC documentation

Now, the A83T and A64 SoC user manuals are available.
Update the documentation to add the links.

An updated version of A83T datasheet is also included now.

Signed-off-by: Icenowy Zheng <icenowy@aosc.xyz>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>