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>