ARM: dts: imx: move nand related property under nand@0

ARM: dts: imx: move nand related property under nand@0

Add child node nand@0 and move NAND related property under it to align
modern nand-controller.yaml.

Fix below CHECK_DTBS warnings:
  arch/arm/boot/dts/nxp/imx/imx6ull-colibri-aster.dtb: nand-controller@1806000 (fsl,imx6q-gpmi-nand): Unevaluated properties are not allowed ('nand-ecc-mode', 'nand-ecc-step-size', 'nand-ecc-strength', 'nand-on-flash-bbt' were unexpected)
        from schema $id: http://devicetree.org/schemas/mtd/gpmi-nand.yaml#

Since 2019 year, commit
(212e496935929 dt-bindings: mtd: Add YAML schemas for the generic NAND options)
NAND related property is preferred located under nand@<n> even though only
one NAND chip supported.

Signed-off-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>