net: mediatek: add support for Airoha AN8855 ethernet switch

Airoha AN8855 is a 5-port gigabit switch with a 2.5G HSGMII CPU port

diff --git a/drivers/net/mtk_eth/Kconfig b/drivers/net/mtk_eth/Kconfig
index 4676847..e8cdf40 100644
--- a/drivers/net/mtk_eth/Kconfig
+++ b/drivers/net/mtk_eth/Kconfig
@@ -32,4 +32,8 @@
 	depends on TARGET_MT7988
 	default y
 
+config MTK_ETH_SWITCH_AN8855
+	bool "Support for Airoha AN8855 ethernet switch"
+	default y if TARGET_MT7981 || TARGET_MT7987
+
 endif # MEDIATEK_ETH