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 <[e-mail]>
Signed-off-by: Alexandre Belloni <[e-mail]>

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

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