When loading U-Boot into RAM over USB protocols using tools such as
tegrarcm or L4T's exec-uboot.sh/tegraflash.py, Tegra's USB device
mode controller is initialized and enumerated by the host PC running
the tool. Unfortunately, these tools do not shut down the USB
controller before executing the downloaded code, and so the host PC
does not "de-enumerate" the USB device. This patch implements optional
code to shut down the USB controller when U-Boot boots to avoid leaving
a stale USB device present.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>