ARM: dts: imx28-xea: Pass the 'model' property

Per root-node.yaml, 'model' is a required property.

Pass it to fix the following dt-schema warning:

imx28-xea.dtb: /: 'model' is a required property
	from schema $id: http://devicetree.org/schemas/root-node.yaml#Signed-off-by: Fabio Estevam <festevam@denx.de>
Fixes: 445ae16a ("ARM: dts: imx28: Add DTS description of imx28 based XEA board")
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/nxp/mxs/imx28-xea.dts

@@ -8,6 +8,7 @@
 #include "imx28-lwe.dtsi"
 
 / {
+	model = "Liebherr XEA board";
 	compatible = "lwn,imx28-xea", "fsl,imx28";
 };