arm64: dts: imx8mm-kontron: Refactor devicetree for OSM-S module and board

arm64: dts: imx8mm-kontron: Refactor devicetree for OSM-S module and board

The OSM spec defines dedicated functions for all pads of the SoM.
Therefore we can assume that carrier board designs stick to these
definitions and extend the SoM devicetree include with matching
default nodes and pinmux settings.

This way we can reduce the overhead and redundancy in the carrier
board devicetrees while still sticking to the policy of separating
board and module description.

Even if the carrier board design deviates slightly from the spec it
can define its own pinmux definitions and use them as necessary or
even disable unused nodes from the SoM devicetree.

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