tegra20: add USB ULPI init code

This adds the required code to set up a ULPI USB port. It is
mostly a port of the Linux ULPI setup code with some tweaks
added for more correctness, discovered along the way of
debugging this.

To use this both CONFIG_USB_ULPI and CONFIG_USB_ULPI_VIEWPORT
have to be set in the board configuration file.

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>