ARM: dts: imx7: Fix memory node duplication

ARM: dts: imx7: Fix memory node duplication

[ Upstream commit 29988e867cb17de7119e971f9acfad2c3fccdb47 ]

Boards based on imx7 have duplicate memory nodes:

- One coming from the board dts file: memory@

- One coming from the imx7s.dtsi file.

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>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>