arm64: dts: imx8mm-emcon: Remove iomuxc pinctrl-names

Remove a 'pinctrl-names' that is not associated with a corresponding
'pinctrl-names'.

This fixes the following schema warning:

imx8mm-emcon-avari.dtb: pinctrl@30330000: 'pinctrl-0' is a dependency of 'pinctrl-names'

Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>