dt-bindings: iio: adc: exynos: Convert Exynos ADC bindings to json-schema

Convert Samsung Exynos Analog to Digital Converter bindings to DT schema
format using json-schema.

This is a direct conversion of existing bindings so it also copies the
existing error in the bindings regarding the requirement of two register
address ranges for certain compatibles.  The inconsistency in binding
was caused by commit fafb37cf ("iio: exyno-adc: use syscon for PMU
register access").

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Rob Herring <robh@kernel.org>