Documentation: dt: rtc: at91rm9200: add clocks property

The RTC needs an input clock, it is the slow clock. It is required as it
will not function without it.
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

Documentation/devicetree/bindings/rtc/atmel,at91rm9200-rtc.txt

@@ -5,6 +5,7 @@ Required properties:
 - reg: physical base address of the controller and length of memory mapped
   region.
 - interrupts: rtc alarm/event interrupt
+- clocks: phandle to input clock.
 
 Example:
 
@@ -12,4 +13,5 @@ rtc@fffffe00 {
 	compatible = "atmel,at91rm9200-rtc";
 	reg = <0xfffffe00 0x100>;
 	interrupts = <1 4 7>;
+	clocks = <&clk32k>;
 };