arm64: dts: imx8mm-kontron: Adjust compatibles, file names and model strings

The official naming includes "SL" (SoM-Line) or "BL" (Board-Line).
By updating we make sure, that we can maintain this more easily in
future and make sure that the proper devicetree can be selected for
the hardware.

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