arm64: dts: imx8mn-evk: Align pin configuration group names with schema

Device tree schema expects pin configuration groups to end with 'grp'
suffix, otherwise dtbs_check complain with a warning like:

    ... do not match any of the regexes: 'grp$', 'pinctrl-[0-9]+'

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