arm64: dts: imx8mm-beacon-baseboard: Remove usbotg2 pinctrl-names

There is no pinctrl description under usbotg2 node, so remove the unneeded
'pinctrl-names' property.

This fixes the following schema warning:

imx8mm-beacon-kit.dtb: usb@32e50000: 'pinctrl-0' is a dependency of 'pinctrl-names'

Signed-off-by: Fabio Estevam <festevam@denx.de>
Acked-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>