ARM: ux500: Enable Kconfig for the generic PM domain

Since ux500 uses the generic PM domain, let's enable the corresponding
Kconfig as long as CONFIG_PM also is enabled.

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