devicetree: bindings: Document supported Allwinner sunxi SoCs

This adds a list of supported Allwinner SoC bindings.
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/devicetree/bindings/arm/sunxi.txt

+Allwinner sunXi Platforms Device Tree Bindings
+
+Each device tree must specify which Allwinner SoC it uses,
+using one of the following compatible strings:
+
+  allwinner,sun4i-a10
+  allwinner,sun5i-a10s
+  allwinner,sun5i-a13
+  allwinner,sun6i-a31
+  allwinner,sun7i-a20
+  allwinner,sun8i-a23
+  allwinner,sun9i-a80