ARM: dts: convert to generic power domain bindings for exynos DT

This patch replaces all custom samsung,power-domain dt
properties with generic power domain bindings and updates
documentation Samsung's devices referring to old binding.

Suggested-by: Kevin Hilman <khilman@kernel.org>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
[javier.martinez@collabora.co.uk: tested on the Exynos5800 Peach Pi Chromebook]
Tested-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Kukjin Kim <kgene@kernel.org>