On the XGS1210-12 the RTL8218D is attached via XGMII. Add this to the
supported list in the DSA driver.
Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://github.com/openwrt/openwrt/pull/16391
Signed-off-by: Robert Marko <robimarko@gmail.com>