airoha: use in-band phylink for RTL8261N USXGMII ports in W1700k

After the standalone Airoha PCS driver and pcs-handle binding, wan (gdm2)
and lan2 (gdm4) netdevs probe but do not pass traffic.

Similar to the Aeonsemi PHY it seems also the RTL PHY needs in-band to the
PCS for USXGMII to work correctly. This still needs to be better
investigated but in the meantime lets apply this workaround to restore
traffic.

Signed-off-by: Daniel Pawlik <pawlik.dan@gmail.com>
[ improve commit description ]
Link: https://github.com/openwrt/openwrt/pull/23383
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>

target/linux/airoha/dts/an7581-w1700k-ubi.dts

@@ -318,9 +318,14 @@
 	nvmem-cell-names = "mac-address";
 };
 
+&pon_pcs {
+	status = "okay";
+};
+
 &gdm2 {
 	status = "okay";
 
+	managed = "in-band-status";
 	phy-handle = <&phy8>;
 	phy-mode = "usxgmii";
 
@@ -329,9 +334,14 @@
 	openwrt,netdev-name = "wan";
 };
 
+&eth_pcs {
+	status = "okay";
+};
+
 &gdm4 {
 	status = "okay";
 
+	managed = "in-band-status";
 	phy-handle = <&phy5>;
 	phy-mode = "usxgmii";