No default serial console on boot.
Fix this by using a 'stdout-path' property that points to the device.

Fixes: c0d9f9ad ("arm64: dts: mt7622: add earlycon to mt7622-rfb1 board")
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Tested-by: Kevin Hilman <khilman@baylibre.com>
[mb: Fix commit message]
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>