realtek: rtl838x: Instantiate auxiliary MDIO bus

realtek: rtl838x: Instantiate auxiliary MDIO bus

Add a disabled node for the auxiliary MDIO bus, used to manage the
RTL8231 expanders. A simple-mfd parent node is added, at the same
(implied) address as the switch@1b000000 node, as the switch drivers
should anyway transistion to MFD subdivices at some point.

Additionally, two pinctrl-single node are added to allow the MDX pins to
be muxed correctly, in case the bootloader leaves these unconfigured.

Signed-off-by: Sander Vanheule <sander@svanheule.net>