USB port LED trigger
  ====================
  
  This LED trigger can be used for signalling to the user a presence of USB device
  in a given port. It simply turns on LED when device appears and turns it off
  when it disappears.
  
  It requires selecting USB ports that should be observed. All available ones are
  listed as separated entries in a "ports" subdirectory. Selecting is handled by
  echoing "1" to a chosen port.
  
  Please note that this trigger allows selecting multiple USB ports for a single
  LED. This can be useful in two cases:
  
  1) Device with single USB LED and few physical ports
  
  In such a case LED will be turned on as long as there is at least one connected
  USB device.
  
  2) Device with a physical port handled by few controllers
  
  Some devices may have one controller per PHY standard. E.g. USB 3.0 physical
  port may be handled by ohci-platform, ehci-platform and xhci-hcd. If there is
  only one LED user will most likely want to assign ports from all 3 hubs.
  
  
  This trigger can be activated from user space on led class devices as shown
  below:
  
    echo usbport > trigger
  
  This adds sysfs attributes to the LED that are documented in:
  Documentation/ABI/testing/sysfs-class-led-trigger-usbport
  
  Example use-case:
  
    echo usbport > trigger
    echo 1 > ports/usb1-port1
    echo 1 > ports/usb2-port1
    cat ports/usb1-port1
    echo 0 > ports/usb1-port1