dt-bindings: iio: adc: exynos: Use defines instead of clock numbers

Make the examples in Exynos ADC 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: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Rob Herring <robh@kernel.org>