rtc: s3c: Document in binding that only s3c6410 needs a src clk

The S3C binding doc says that the RTC and RTC source clocks are required
but the S3C driver supports different HW IP and only the s3c6410 needs a
source clock.

Fix the binding explaining that the source clock is only needed for the
s3c6410-rtc compatible controller.

Reported-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

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

@@ -15,9 +15,10 @@ Required properties:
   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.
+          clock and in the case of a s3c6410 compatible controller, also
+          a source clock.
+- clock-names: Must contain "rtc" and for a s3c6410 compatible controller,
+               a "rtc_src" sorted in the same order as the clocks property.
 
 Example: