This adds basic USB support for port 0. The other port is not supported
yet.

Tegra2 (SeaBoard) # usb start
(Re)start USB...
USB:   Register 10011 NbrPorts 1
USB EHCI 1.00
scanning bus for devices... 5 USB Device(s) found
       scanning bus for storage devices... 1 Storage Device(s) found
Tegra2 (SeaBoard) # ext2load usb 0:3 10000000 /boot/vmlinuz
Loading file "/boot/vmlinuz" from usb device 0:3 (ROOT-A)
2932976 bytes read

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>