ARM: dts: meson8b: mxq: add the /chosen/stdout-path property

Support for this board is currently very limited. To debug any potential
issues on this board 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>