realtek: move RTL8231 definitions to board files

realtek: move RTL8231 definitions to board files

The RTL8231 is an external chip, and not part of the SoC. That means
it is more appropriate to define it in the board specific (base) files,
instead of the DT include for the SoC itself.

Moving the RTL8231 definition also ensures that boards with no GPIO
expander, or an alternative one, don't have a useless gpio1 node label
defined.

Tested on a Netgear GS110TPPv1.

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