dt-bindings: iio: adc: exynos: Remove old requirement of two register address ranges

Commit fafb37cf ("iio: exyno-adc: use syscon for PMU
register access") changed the Exynos ADC driver so the PMU syscon
phandle is required instead of second register address space.  The
bindings were not updated so fix them now.

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