ARM: dts: imx51-zii-rdu1: limit usbh1 to full-speed

On RDU1, imx51 usbh1 interface is either not used, or used via external
block that breaks USB2 signalling.

To keep things working if high-speed device gets connected to that
block, use ChipIdea feature to limit port to full speed.

Signed-off-by: Nikita Yushchenko <nikita.yoush@cogentembedded.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>