usb: core: read USB ports from DT in the usbport LED trigger driver

This uses DT info to read relation description of LEDs and USB ports. If
DT has properly described LEDs, trigger will know when to turn them on.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>