rtc: s3c: Document required clocks in the DT binding

The S3C Real Time Clock driver requires the clock and source clock to
be defined in the device node but that requirement is not documented.

Signed-off-by: Javier Martinez Canillas <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>

Documentation/devicetree/bindings/rtc/s3c-rtc.txt

···
   interrupt number is the rtc alarm interrupt and second interrupt number
   is the rtc tick interrupt. The number of cells representing a interrupt
   depends on the parent interrupt controller.
+- clocks: Must contain a list of phandle and clock specifier for the rtc
+          and source clocks.
+- clock-names: Must contain "rtc" and "rtc_src" entries sorted in the
+               same order as the clocks property.
 
 Example:
 
···
 		compatible = "samsung,s3c6410-rtc";
 		reg = <0x10070000 0x100>;
 		interrupts = <44 0 45 0>;
+		clocks = <&clock CLK_RTC>, <&s2mps11_osc S2MPS11_CLK_AP>;
+		clock-names = "rtc", "rtc_src";
 	};