In order to be able to define the external GPIO controller on an
emulated MDIO bus, move the controller definition outside of the main
GS1900 include for RTL838x-based devices.
Additionally, a new DTSI is provided defining the RTL8231 on the
emulated MDIO bus.
Signed-off-by: Sander Vanheule <sander@svanheule.net>
