can: add combined rx/tx LED trigger support

Add <ifname>-rxtx trigger, that will be activated both for tx
as rx events. This trigger mimics "activity" LED for Ethernet
devices.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>