leds: trigger: Introduce a NETDEV trigger

This commit introduces a NETDEV trigger for named device
activity. Available triggers are link, rx, and tx.

Signed-off-by: Ben Whitten <ben.whitten@gmail.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>