ARM: dts: mmp2: rename the USB PHY node

This device is not an OTG phy, it's a regular USB HS phy. Follow the
generic node name recommendation, and rename it to "usb-phy".

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Acked-by: Pavel Machek <pavel@ucw.cz>