ARM: dts: meson6: atv1200: add the /chosen/stdout-path property

Support for Meson6 SoCs is currently very limited. It's often unclear
why such a device does not boot. To debug this the "earlycon" kernel
parameter can be used (without any arguments). However, this requires
the board to define a /chosen/stdout-path property in it's .dts.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>