ARM: i.MX28: dts: rename usbphy pin names

The pins have nothing todo with the phy layer. We better rename them, so
it gets clear they are routed to the ehci core not to any phy.

Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>