leds: trigger: Introduce a kernel panic LED trigger

This commit introduces a new LED trigger which allows to configure
a LED to blink on a kernel panic (through panic_blink).

Notice that currently the Openmoko FreeRunner (GTA02) mach code
sets panic_blink to blink a hard-coded LED. The new trigger is
meant to introduce a generic mechanism to achieve this.

Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>