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The samsung,power-domain property is obsolete since commit 0da65870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>

Documentation/devicetree/bindings/display/exynos/exynos7-decon.txt

@@ -33,7 +33,7 @@ Required properties:
 - i80-if-timings: timing configuration for lcd i80 interface support.
 
 Optional Properties:
-- samsung,power-domain: a phandle to DECON power domain node.
+- power-domains: a phandle to DECON power domain node.
 - display-timings: timing settings for DECON, as described in document [1].
 		Can be used in case timings cannot be provided otherwise
 		or to override timings provided by the panel.