[][Add mt7981 dts/dtsi]

[Description]
Add mt7981 dts/dtsi

[Release-log]
N/A

Change-Id: Ie5dd4f5acb2ad11ed3626506be3f93db395b7b91
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/5188437
16 files changed