Documentation: devicetree: Remove Exynos bindings from kernel

This is a preparation change for merging android-4.19-q.73 into
msm-4.19 branch.

Remove Samsung Exynos-adc bindings from kernel added by [1] and [2].
[1] and [2] are conflicting against [3] in msm-4.19.

[1] 4e516b72("dt-bindings: iio: adc: exynos-adc: Add S5PV210 variant")
[2] 0d7f329e("iio: adc: exynos-adc: Use proper number of channels for Exynos4x12")
[3] 728aba9a("Documentation: devicetree: Remove devicetree bindings from kernel")

Change-Id: Icc39b44738516b76dcab3257657571b8d6774bd1
Signed-off-by: Ivaylo Georgiev <irgeorgiev@codeaurora.org>