realtek: drop old RTL8231 driver

The old RTL8231 driver integrated the MDIO bus access with the GPIO
control ops, making this driver not very portable to newer platforms.
It depended on the SoC ID instead of the compatible to determine the
MDIO access register, further complicating portability.

A new MFD driver is now available, which offers proper pin config as
well as optional LED support, which can work on any (bitbanged) MDIO
bus. Now that all devices have been migrated, we can drop the old code.

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