ARM: tegra: enable USB device mode and UMS on some boards

For each of Jetson TK1, Venice2, and Beaver:

- Enable the first USB controller in DT, and describe its configuration.

- Enable USB device/gadge support. This allows the user to type e.g.
  "ums 0 mmc 0" at the command-line to cause U-Boot to act a USB device
  implementing the USB Mass Storage protocol, and expose MMC device 0
  that way.

This allows a host PC to mount the Tegra device's MMC, partition it, and
install a filesystem on it.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
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Note: This depends on a bunch of changes to the Tegra EHCI driver and
U-Boot UMS (USB Mass Storage) code, which are likely to be taken through
the USB maintainer tree. We should hold off until those commits show up
in the main U-Boot repo before applying this patch.