dt-bindings: Consolidate Exynos SoC bindings

Exynos SoC Device Tree bindings are spread over arm/exynos/ and
arm/samsung/ directories. There is no need for that separation and it
actually confuses. Put power domain bindings under power/ and
remaining samsung-boards.txt under arm/samsung/.
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>

Documentation/devicetree/bindings/iommu/samsung,sysmmu.txt

@@ -47,7 +47,7 @@ Required properties:
 - clocks: Required if the System MMU is needed to gate its clock.
 - power-domains: Required if the System MMU is needed to gate its power.
 	  Please refer to the following document:
-	  Documentation/devicetree/bindings/arm/exynos/power_domain.txt
+	  Documentation/devicetree/bindings/power/pd-samsung.txt
 
 Examples:
 	gsc_0: gsc@13e00000 {