Fold the usb2.txt documentation about specifying which physical
port a USB device should use into usb.rst.
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20210728141457.14825-4-peter.maydell@linaro.org>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>