tegra: usb: move controller init into start_port

There is no need to init a USB controller before the upper layers indicate
that they are actually going to use it.

board_usb_init now only parses the device tree and sets up the common pll.

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>