usb: Add new command to set USB 2.0 port test modes

This patch adds a new 'usb test' command, that will set a port to a USB
2.0 test mode (see USB 2.0 spec 7.1.20). It supports all five test modes
on both downstream hub ports and ordinary device's upstream ports. In
addition, it supports EHCI root hub ports.

Signed-off-by: Julius Werner <jwerner@chromium.org>