USB: add the usbfs devices file to debugfs

People are very used to the devices file in usbfs.  Now that we have
moved usbfs to be an "embedded" option only, the developers miss the
file, they had grown quite attached to it over all of these years.  This
patch brings it back and puts it in the usb debugfs directory, so that
the developers don't feel sad anymore.

Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>