leds: Add leds-wrap default-trigger

The power led is normally lit after boot, let's use the default-on
trigger as the default trigger for it.  This gets the initial brightness
value right and being on is the default behaviour we expect for a power
led.

Signed-off-by: Sven Wegener <sven.wegener@stealer.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>