Per memory.yaml, 'device_type' is a required property.

Pass it to fix the following dt-schema warning:

imx28-xea.dtb: /: memory@40000000: 'device_type' is a required property
	from schema $id: http://devicetree.org/schemas/memory.yaml#Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>