arm64: dts: mt7622: fix no more console output on BPI-R64 board

Fix this by using a 'stdout-path' property that points to the device.

Fixes: 0b6286dd ("arm64: dts: mt7622: add bananapi BPI-R64 board")
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>