MIPS: BCM47XX: Use "timer" trigger for status LEDs

Some devices have power LED as well as status LED. The second one is
used to show the firmware is up and running. Set "timer" trigger for
such LEDs.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/6312/