ARM: dts: imx: Change gpmi nand node name to nand-controller

Change i.MX SoCs nand node name from "gpmi-nand" to "nand-controller" to
be compliant with yaml schema, it requires the nodename to be
"nand-controller".

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>