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>