Tegra SOC USB controllers

The device node for a USB controller that is part of a Tegra
SOC is as described in the document "Open Firmware Recommended
Practice : Universal Serial Bus" with the following modifications
and additions :

Required properties :
 - compatible : Should be "nvidia,tegra20-ehci" for USB controllers
   used in host mode.
 - phy_type : Should be one of "ulpi" or "utmi".
 - nvidia,vbus-gpio : If present, specifies a gpio that needs to be
   activated for the bus to be powered.
