clk: sunxi: document new A31 USB clock compatible

Support for the USB gates and resets on A31 has been recently added
using a new compatible, so let's document it here.
Signed-off-by: Emilio López <emilio@elopez.com.ar>

Documentation/devicetree/bindings/clock/sunxi.txt

@@ -41,6 +41,7 @@ Required properties:
 	"allwinner,sun7i-a20-gmac-clk" - for the GMAC clock module on A20/A31
 	"allwinner,sun4i-a10-usb-clk" - for usb gates + resets on A10 / A20
 	"allwinner,sun5i-a13-usb-clk" - for usb gates + resets on A13
+	"allwinner,sun6i-a31-usb-clk" - for usb gates + resets on A31
 
 Required properties for all clocks:
 - reg : shall be the control register address for the clock.