dt-bindings: video: exynos7-decon: Remove obsolete samsung,power-domain property

The samsung,power-domain property is obsolete since commit 0da65870
("ARM: dts: convert to generic power domain bindings for exynos DT").
Replace it with generic one.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Acked-by: Rob Herring <robh@kernel.org>