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

The baseboard for the Kontron N6310 SoM is also used for other SoMs
such as N6311 and N6411. In order to share the code, we move the
definitions of the baseboard to a separate dtsi file.

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