realtek: make all link speeds work for RTL8221B in Zyxel XGS1210-12 rev B1

realtek: make all link speeds work for RTL8221B in Zyxel XGS1210-12 rev B1

SGMII only works correctly on this device if inband auto-negotiation is
enabled. Configure the PHY for SGMII and in-band mode in the device tree
to make this happen.

For 2.5G link speeds the PHY will still switch to 2500Base-X without AN.

The same configuration also works on RTL8226, so it is fine to apply
this change to the A1 revision of XGS1010-12/XGS1210-12 as well.

Signed-off-by: Jan Hoffmann <jan@3e8.eu>
Link: https://github.com/openwrt/openwrt/pull/21605
Signed-off-by: Robert Marko <robimarko@gmail.com>