arm64: dts: imx8mn-evk: remove orphaned pinctrl-names property

The "pinctrl-names" property in iomux node does not make sense on its
own (without "pinctrl-X").

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