document: dt: add binding for Hi3660 SoC

Add binding for hisilicon Hi3660 SoC and HiKey960 Board.
Signed-off-by: Chen Feng <puck.chen@hisilicon.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Wei Xu <xuwei5@hisilicon.com>

Documentation/devicetree/bindings/arm/hisilicon/hisilicon.txt

 Hisilicon Platforms Device Tree Bindings
 ----------------------------------------------------
+Hi3660 SoC
+Required root node properties:
+	- compatible = "hisilicon,hi3660";
+
 Hi4511 Board
 Required root node properties:
 	- compatible = "hisilicon,hi3620-hi4511";