arm: dts: mediatek: add basic support for MT7629 SoC

This adds basic support for MT7629 reference board.

Tested-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>