ARM: dts: imx: remove redundant intermediate node in pinmux hierarchy

ARM: dts: imx: remove redundant intermediate node in pinmux hierarchy

Remove the redundant intermediate node between the pinmux and group nodes,
and add the missing "grp" suffix to the group node names.

Fix below CHECK_DTBS warnings:
arm/boot/dts/nxp/imx/imx27-apf27dev.dtb: iomuxc@10015000 (fsl,imx27-iomuxc): Unevaluated properties are not allowed ('imx27-apf27', 'imx27-apf27dev' were unexpected)
        from schema $id: http://devicetree.org/schemas/pinctrl/fsl,imx27-iomuxc.yaml

Signed-off-by: Frank Li <Frank.Li@nxp.com>