arm: dts: mt7623: enable three available UARTs on bananapi-r2

arm: dts: mt7623: enable three available UARTs on bananapi-r2

On bpi-r2 board, totally there're four UARTs which we usually called
uart[0-3] helpful to extend slow-I/O devices. Among those ones, uart2 has
dedicated pin slot which is used to console log. uart[0-1] appear at the
40-pins connector and uart3 has no pinout, but just has test points (TP47
for TX and TP48 for RX, respectively) nearby uart2, but we don't enable
uart3 in the patch. The missing pinctrl is also being supplemented for
those newly added devices.

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