ARM: dts: imx: make MMDC node name generic

Node name should be generic, so use "memory-controller"
instead of "mmdc" for MMDC node name, also remove "mmdc"
label for platforms with single MMDC node.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>