ARM: dts: vf: Fix memory node duplication

Boards based on vf500/vf600 have duplicate memory nodes:

- One coming from the board dts file: memory@

- One coming from the vf500.dtsi/vf610m4.dtsi files.

Fix the duplication by removing the memory node from the dtsi file
and by adding 'device_type = "memory";' in the board dts.

Reported-by: Rob Herring <robh@kernel.org>
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>