Add a driver that exposes the auxiliary busses, used for the RTL8231
expanders, as a proper MDIO controller. The device must be instantiated
under an MFD device, so the driver should also be compatible with SoC
managed by an external CPU via SPI.
Leave the driver disabled in builds until required.
Signed-off-by: Sander Vanheule <sander@svanheule.net>
