usb: core: Introduce a USB port LED trigger

This commit adds a new trigger responsible for turning on LED when USB
device gets connected to the selected USB port. This can can useful for
various home routers that have USB port(s) and a proper LED telling user
a device is connected.

The trigger gets its documentation file but basically it just requires
enabling it and selecting USB ports (e.g. echo 1 > ports/usb1-1).

There was a long discussion on design of this driver. Its current state
is a result of picking them most adjustable solution as others couldn't
handle all cases.

1) It wasn't possible for the driver to register separated trigger for
   each USB port. Some physical USB ports are handled by more than one
   controller and so by more than one USB port. E.g. USB 2.0 physical
   port may be handled by OHCI's port and EHCI's port.
   It's also not possible to assign more than 1 trigger to a single LED
   and implementing such feature would be tricky due to syncing triggers
   and sysfs conflicts ...