dt-bindings: arm: fsl: Add Kontron i.MX8M Mini SoMs and boards

Add entries for the SoMs and boards based on i.MX8MM from Kontron
Electronics GmbH.

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