ARM: ux500: Enable system suspend with WFI support

When building for CONFIG_SUSPEND, add the platform suspend callbacks
to enable system suspend for ux500.

At this initial step, only WFI state is supported, which is reached
for both PM_SUSPEND_MEM and PM_SUSPEND_STANDBY.

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