arm64: dts: verdin-imx8mp: dahlia: mark usb_2 permanently attached

As both Dahlia and the Verdin Development Board have on-carrier
permanently attached USB hubs mark Verdin USB_2 as such.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>