[][dts: mt7986: port correct dts]

[Description]
Fix dts:
1. Refine all files' indentation.
2. Seperate snfi & spim dts files.
3. Remove mt7986b-mt7976-ax6000-rfb4 dts and related builds.

[Release-log]
N/A

Change-Id: I605ac105999c53858c86270f7468c119c41e1533
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/5053660
18 files changed