mtd: nand: sunxi: update DT bindings

Document dmas and dma-names properties.
Signed-off-by: NBoris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: NRob Herring <robh@kernel.org>
Acked-by: NMaxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: NBoris Brezillon <boris.brezillon@free-electrons.com>

Documentation/devicetree/bindings/mtd/sunxi-nand.txt

@@ -11,6 +11,10 @@ Required properties:
     * "ahb" : AHB gating clock
     * "mod" : nand controller clock
 
+Optional properties:
+- dmas : shall reference DMA channel associated to the NAND controller.
+- dma-names : shall be "rxtx".
+
 Optional children nodes:
 Children nodes represent the available nand chips.