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: NIcenowy Zheng <icenowy@aosc.xyz>
Acked-by: NChen-Yu Tsai <wens@csie.org>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/arm/sunxi/README

@@ -73,4 +73,13 @@ SunXi family
     * Octa ARM Cortex-A7 based SoCs
       - Allwinner A83T
         + Datasheet
-          http://dl.linux-sunxi.org/A83T/A83T_datasheet_Revision_1.1.pdf
+          https://github.com/allwinner-zh/documents/raw/master/A83T/A83T_Datasheet_v1.3_20150510.pdf
+        + User Manual
+          https://github.com/allwinner-zh/documents/raw/master/A83T/A83T_User_Manual_v1.5.1_20150513.pdf
+
+    * Quad ARM Cortex-A53 based SoCs
+      - Allwinner A64
+        + Datasheet
+          http://dl.linux-sunxi.org/A64/A64_Datasheet_V1.1.pdf
+        + User Manual
+          http://dl.linux-sunxi.org/A64/Allwinner%20A64%20User%20Manual%20v1.0.pdf