drivers: pinctrl: Allow keeping pinctrl sleep state

When PM or PM_DEVICE is enabled pinctrl sleep state is using for
device suspension. However, there are cases where power management is
not used but we still want to be able to put pins to sleep state, e.g.
device deinit.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>