ARM: dts: imx6ul-kontron-n6x1x: Add 'chosen' node with 'stdout-path'

The Kontron N6x1x SoMs all use uart4 as a debug serial interface.
Therefore we set it in the 'chosen' node.

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>