Enable pin controller drivers to be modules

The pinctrl subsystem does not really support modules: different devices take struct pinctrl * handles and these live as long as the devices are probed. However it would be possible to orphan the handles (have no pinctrl driver backend associated with them at all) by refactoring the pinctrl core to take a lock around the state in struct pinctrl and introduce code to dynamically add and remove states as pin controllers are registered/unregistered.

This feature may be good to reduce total kernel footprint on devices with non-strict boot time requirements that want to load as much of the required drivers as possible as modules from initramfs or a root filesystem.