dt-bindings: arm: Document emtrion emCON-MX6 bindings

Document the compatible strings for emtrion emCON-MX6 SoM's.
Signed-off-by: Jan Tuerk <jan.tuerk@emtrion.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

Documentation/devicetree/bindings/arm/emtrion.txt

+Emtrion Devicetree Bindings
+===========================
+
+emCON Series:
+-------------
+
+Required root node properties
+	- compatible:
+	- "emtrion,emcon-mx6", "fsl,imx6q"; 		: emCON-MX6D or emCON-MX6Q SoM
+	- "emtrion,emcon-mx6-avari", "fsl,imx6q";	: emCON-MX6D or emCON-MX6Q SoM on Avari Base
+	- "emtrion,emcon-mx6", "fsl,imx6dl"; 		: emCON-MX6S or emCON-MX6DL SoM
+	- "emtrion,emcon-mx6-avari", "fsl,imx6dl";	: emCON-MX6S or emCON-MX6DL SoM on Avari Base