ARM: dts: imx6ul-kontron-n6310: Move common SoM nodes to a separate file

The Kontron N6311 and N6411 SoMs are very similar to N6310. In
preparation to add support for them, we move the common nodes to a
separate file imx6ul-kontron-n6x1x-som-common.dtsi.

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