ARM: at91/dt: rm9200: fix usb clock definition

The atmel,clk-divisors property is taking 4 divisors, if less are
provided, the clock registration will fail.
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>

arch/arm/boot/dts/at91rm9200.dtsi

@@ -149,7 +149,7 @@ mck: masterck {
 				usb: usbck {
 					compatible = "atmel,at91rm9200-clk-usb";
 					#clock-cells = <0>;
-					atmel,clk-divisors = <1 2>;
+					atmel,clk-divisors = <1 2 0 0>;
 					clocks = <&pllb>;
 				};