ARM: ux500: cpuidle: Instantiate the driver from platform device

To break the dependency on the "id.h" file we move the cpuidle driver
to a platform device. Now we only call the probe() on this driver if
we find a corresponding platform device (which is spawned from the
PRCMU MFD driver).

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>