Implement a new method to retrieve the gpiobus reference from a GPIO
controller.

The gpiobus is responsible to keep track of the used pins and serialize
the access to pins.

Some of these features are important to devices that do not descend
directly from gpiobus and as such cannot make use of its features (one
classic example is gpioc that is attached to the GPIO controller and could
not, until now, make use of the gpiobus locking).