ramips: set usb led trigger via devicetree

Assign the usbdev trigger via devicetree for all subtargets and drop
the userspace handling of the usb leds.

With the change all usb ports are triggering the usb led instead of
only usb 1.1 XOR usb 2.0 XOR usb 3.0 as it was before.

Signed-off-by: Mathias Kresin <dev@kresin.me>