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>

Documentation/arm/sunxi/README

@@ -58,4 +58,5 @@ SunXi family
 
     * Quad ARM Cortex-A15, Quad ARM Cortex-A7 based SoCs
       - Allwinner A80
-        + Not Supported
\ No newline at end of file
+        + Datasheet
+	  http://dl.linux-sunxi.org/A80/A80_Datasheet_Revision_1.0_0404.pdf