ARM: dts: exynos: Remove not accurate secondary ADC compatible

The Exynos3250 ADC has its own compatible because of differences from
other Exynos SoCs.  Therefore it is not entirely compatible with
samsung,exynos-adc-v2.  Remove the samsung,exynos-adc-v2.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>