arm64: dts: mt7622: fix no more console output on rfb1

arm64: dts: mt7622: fix no more console output on rfb1

BugLink: https://bugs.launchpad.net/bugs/1837477
[ Upstream commit 6c05946e349d92f527d98644fbc9c41f06312c00 ]

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

Fixes: c0d9f9ad4f76 ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>