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 : For Tegra20, must contain "nvidia,tegra20-ehci".
   For Tegra30, must contain "nvidia,tegra30-ehci".  Otherwise, must contain
   "nvidia,<chip>-ehci" plus at least one of the above, where <chip> is
   tegra114, tegra124, tegra132, or tegra210.
 - nvidia,phy : phandle of the PHY that the controller is connected to.
 - clocks : Must contain one entry, for the module clock.
   See ../clocks/clock-bindings.txt for details.
 - resets : Must contain an entry for each entry in reset-names.
   See ../reset/reset.txt for details.
 - reset-names : Must include the following entries:
   - usb

Optional properties:
 - nvidia,needs-double-reset : boolean is to be set for some of the Tegra20
   USB ports, which need reset twice due to hardware issues.
