ARM: ux500: Add DT node for ux500 PM domains

Add a DT node for the ux500 PM domains. Follow the DT semantics of the
generic PM domain.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>