realtek: Add virtual MDIO bus on rtl838x

realtek: Add virtual MDIO bus on rtl838x

Some RTL8380M-based devices have been around for a long time and use an
early A revision of the RTL8380M SoC. This revision has an issue with
the auxiliary MDIO controller, causing it to malfunction. This may lead
to device reboots when the controller is used.

Provide a bit-banged MDIO bus, which muxes the auxiliary MDIO pins to
their GPIO function. Although this will result in lower performance,
there should otherwise be no functional differences.

Link: https://github.com/openwrt/openwrt/issues/9534
Signed-off-by: Sander Vanheule <sander@svanheule.net>