dt-bindings: iio: adc: exynos-adc: do not require syscon on S5Pv210

dt-bindings: iio: adc: exynos-adc: do not require syscon on S5Pv210

The ADC in S5Pv210 does not have ADC phy registers in separate block for
which syscon would be needed.  Remove this requirement to fix dtbs_check
warnings like:

  arch/arm/boot/dts/s5pv210-fascinate4g.dt.yaml: adc@e1700000: 'samsung,syscon-phandle' is a required property

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huwei.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200910161933.9156-2-krzk@kernel.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>