kirkwood: set sata/usb led trigger for NSA3xx

these two devices have a Sata led for each sata port.
These leds must be controlled separately by a special
sata led trigger already used in oxnas target.

Both these devices have a single USB led, and to keep
consistent behaviour with the Sata leds that show
sata activity, this led uses usb-host trigger
to show usb activity.

Signed-off-by: Alberto Bursi <alberto.bursi@outlook.it>