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>