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 <[e-mail]>
Reviewed-by: Javier Martinez Canillas <[e-mail]>
Signed-off-by: Kukjin Kim <[e-mail]>

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

···
 - 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 {