[][kernel][mt7988][eth][dts: add gphy nodes under slave mdio]

[Description]
Add gphy nodes under slave mdio

[Release-log]
N/A

Change-Id: I18de6eb4e0ffca7a8fac80ae88683394dddfe5e6
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/6660300
Build: srv_hbgsm110
7 files changed