ARM: dts: imx28-m28: Align GPMI NAND node name with schema

Device tree schema expects NAND controller to be named
"nand-controller", otherwise dtbs_check complain with a warning like:

    arch/arm/boot/dts/imx28-eukrea-mbmx283lc.dt.yaml: gpmi-nand@8000c000:
        $nodename:0: 'gpmi-nand@8000c000' does not match '^nand-controller(@.*)?'

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>