leds/trigger/cpu: Add LED trigger for all CPUs aggregated

Currently there is one CPU led trigger per cpu ('cpu0', 'cpu1', ...)

This patch adds a new trigger, 'cpu', with brightness proportional to
the number of active CPUs.

If multiple brightness levels aren't supported on the LED,
it effectively indicates if there is any CPU active.

This is particularly useful on tiny linux boards with more CPU cores than LED pins.

Signed-off-by: Paulo Costa <me@paulo.costa.nom.br>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>