device: move device power management state into common dynamic state

This avoids the need for distinct object that uses flash to store its
initializer.  Instead the state is initialized when the kernel is
starting up, before anything can reference it.  In future refactoring
the PM state could be accessed directly without storing an extra
pointer in the static device state.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>