realtek: make use of serdes helper for TP-Link SG2xxx

Use the new INTERNAL_PHY_SDS() helper to describe the SFP ports. With
this change the driver now knows that ports 24/26 are driven by serdes
4/5.

For the RTL838x devices this is currently only an additional information
for the mdio bus. It is not evaluated further because everything is
hardcoded.

Signed-off-by: Markus Stockhausen <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/18851
Signed-off-by: Robert Marko <[email protected]>

target/linux/realtek/dts/rtl8380_tplink_sg2xxx.dtsi

@@ -152,8 +152,8 @@
 		INTERNAL_PHY(14)
 		INTERNAL_PHY(15)
 
-		INTERNAL_PHY(24)
-		INTERNAL_PHY(26)
+		INTERNAL_PHY_SDS(24, 4)
+		INTERNAL_PHY_SDS(26, 5)
 	};
 };