arm64: dts: zii-ultra: limit USB ports to USB2 speed

The internal USB is connected to a USB2 hub, the front-panel USB
is wired directly, but does not support USB3 speeds electrically.
Limit both ports accordingly.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>