board: mediatek: mt7629: Migrate to OF_UPSTREAM

Move differences in DT files between upstream Linux DT and U-Boot DT to
mt7629-rfb-u-boot.dtsi.
Remove old copies of mt7629-related clock bindings, .dts, and .dtsi files.
Update defconfig to switch the whole mt7629 SoC to use OF_UPSTREAM.

6 files changed