arm64: dts: mt7622: add earlycon to mt7622-rfb1 board

Add earlycon to mt7622-rfb1 as to know what was going on when a certain
fault is happening at the early initialization stage.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>