dt-bindings: rtc: s3c: Use defines instead of clock numbers

Make the examples in S3C RTC bindings more readable and bring them
closer to real DTS by using defines for clocks.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Rob Herring <robh@kernel.org>