arm: dts: mt7623: cleanup the mt7623n rfb uart nodes

This patch does a cleanup of the uart nodes in the dts file of the RFB. It
adds aliases, enables 2 more uarts and explicitly sets the uart mode of the
console.

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